Time, tense and aspect: overview and index

hour glass

If you feel comfortable with the following concepts ...

time, tense and aspect
perfect, progressive, continuous, habitual, iterative, durative and prospective aspects
the distinction between progressive and continuous aspects
the distinction between stative and dynamic verb use
perfective, perfect and imperfect verb forms
telic and atelic clauses
finite and non-finite verb forms

... then feel free to use this index to go to the area that interests you.

Talking about the Present	Talking about the Past	Talking about the Future	Talking about Always
There is also a guide to:
Teaching tense and aspect

Otherwise, read on ...

Clearing some conceptual decks

Before we can look at tense forms, meaning and use in English, we have to get four things clear:

Time:

This is a non-linguistic concept of when an action or a state is set. It is non-linguistic because the relationship between time and grammatical form is often unclear with past forms used to speak about the future, future forms appearing in the past and so on.
Traditionally, we have three choices:

Now: the action / state is set in the present, e.g.:
    I am on the train
    Please come with me
    If you could come with me, it might help
Past: the action / state is set before the present, e.g.:
    She arrived late
    I was to have attended the meeting but couldn't be there
    Where would he have gone next?
Future: the action / state is set after the present, e.g.:
    My train leaves soon
    Do you think she will be at the party?
    I will have been here for years by then
but we can add a fourth and that concerns Always: the action / state is set in the past, the present and the future, e.g.:
    I like apples
    She is working too hard these days
    We tend to go to bed quite late
This expression of time in a language is often called timeless because it is not, obviously tied to a specific time.
Habitual expressions fall into this category along with expressions of liking and disliking which are assumed to apply to the past, present and future.

Tense:

No, not that sort.
This is the name we give to the form the verb takes. It refers to the form of the verb and the grammatical function of any primary auxiliary verbs involved. In English it is often averred that there are just two:

past tense: e.g., made rather than make
present tense: e.g., comes rather than came
but we'll add a third because English most certainly does exhibit future forms (such as will go rather than goes) even if it has no 'proper' future tense signalled by an alteration in the form of the main verb as many languages do (and many don't)

Aspect:

This refers to how the user of the language views the event or how the event is experienced. There are many of these and they are signalled in languages in a bewildering number of ways. For the purposes of analysing English verb form, meaning and use, we'll focus on:

perfect: relating two times (past to pre-past, present to pre-present, future to post-future). An alternative way to see this (explained below) is that perfect aspects set times within other times.
progressive: ongoing
continuous: current
habitual: routine
iterative: repeated
durative: long lasting
prospective: looking forward

Modality:

The expression of tenses is often allied to modality because, by definition, the future is uncertain, we may speculate about the past and we may not be sure of our facts even in the present. In all tenses, we can speak of our concerns with regard to willingness, ability, uncertainty, likelihood, permissibility or duty.
Modality concerns the view that the speaker / writer wishes to convey regarding four key concepts:

the truth or otherwise of a proposition: relating to how the user of the languages perceives the likelihood of something being the case. For example, expressing doubt or certainty.
This is known as epistemic modality, from the Greek for knowledge. E.g.:
    That might have been her father
    She'll probably be late
    I expect he's upstairs
etc.
obligation: relating to the language user's understanding of whether an obligation or its lack is intended to be expressed. The obligation can vary in strength and be legal, ethical, moral or advisory.
For example, there is an external obligation, an internal sense of duty, a lack of obligation or an expression of advice.
This is known as deontic modality from the Greek for being a needed duty. E.g.:
    They shouldn't have told you that
    It would be better if you didn't
    You ought to stay
    They don't have to go
ability and willingness: this expresses the language user's view concerning how possible it is for someone or something to do what the verb suggests or how willing they are to do it.
Because of the sense of power and willingness, this is called dynamic modality from the Greek for power. E.g.:
    I couldn't safely have driven any faster
    I won't be able to come
    I will get it done for you
    I can't help, I'm afraid
necessity: concerns the language user's view of a sense of inevitability so any statement which implies an unavoidable logical conclusion falls into this category. Essential truths about the workings of the natural world or concepts of definitions of states fall into this category.
It is called alethic modality from the Greek for truth.
    If it was the colour you described, it must have been a buzzard
    Even on Pluto, Newton's laws will apply
    Two and two must make four

Here's the cut-out-and-keep guide to summarise this.

summary

Modality is not included in what follows because that is covered elsewhere in the section dedicated to modality which you can go to here.

Some examples to make this clear

Example	Time	Tense	Aspect
That'll be the postman	Now	Future simple	Continuous
That'll be the postman	he is here now	signalled by the auxiliary, ''ll' + bare infinitive	he is standing outside the door
I have always arrived on Sunday	Always	Present perfect	Habitual
I have always arrived on Sunday	signalled by the adverb	signalled by the form of the verb and the auxiliary	this is a routine
He thinks so	Now / Always	Present simple	Continuous
He thinks so	this is, was and will be his state of mind	signalled by the -s inflexion	this is a current state
John is writing a book	Now	Present	Progressive
John is writing a book	this is one of his current occupations	signalled by the auxiliary, is + -ing	this is an ongoing or background action
John is always banging on about it	Always	Present	Iterative
John is always banging on about it	signalled by the adverb	signalled by the auxiliary, is + -ing	a repeated action
Murray hits a backhand down the line	Now	Present	Progressive
Murray hits a backhand down the line	commentary on current actions	signalled by -s	describing an action in progress
I have finished	Present	Present perfect	Perfect
I have finished	describing a current situation	signalled by the form of the verb and the auxiliary, have	the present with an embedded past idea
I leave tomorrow	Future	Present	Simple
I leave tomorrow	signalled by the adverb	signalled by base form of the verb with a Ø inflexion	a one-off event
I was staying at the Ritz then	Past	Past	Durative
I was staying at the Ritz then	signalled by the adverb	signalled by the auxiliary, was + -ing	the state went on for some time
She lived in France as a child	Past	Past simple	Durative
She lived in France as a child	signalled by the prepositional time phrase	signalled by the regular past-tense form	only certain verbs can be used with this meaning
He's going to fall	Future	Present	Prospective
He's going to fall	signalled by the context	signalled by the verb forms	current opinion of an event after now
What will you do if she doesn't arrive?	Future	Present	Simple
What will you do if she doesn't arrive?	signalled by the if-structure	signalled by -s	a one-off event
What would you do if she didn't arrive?	Future	Past	Simple
What would you do if she didn't arrive?	signalled by the if-structure	signalled by the auxiliary, did + n't + bare infinitive	a one-off event

We can see from even this short list, that time, tense and aspect do not exist in a one-to-one relationship. Tenses are not, formally, tied to time and aspect is not always signalled by formal changes in the verb.

The starting point

Before we can consider how English encodes notions of time, we have to decide what sort of tense system it actually has.
This may not be a question many practising teachers ask but it is important.
This site works from the premise that English has a very simple tense inflexion system but a complex aspectual tense system.
What do we mean by this?

Well, if, for example, we take the past tense of a verb in English such as walk, we find that the only change we have to make to the base form of the verb is to add -ed to the verb and that gives us walked, of course. If we form what we are pleased to call a simple future in English, we simply add an auxiliary verb and arrive at will walk. Those are the only changes we make to simple aspects.
Even when we make a progressive form, we can see from what follows that very small changes are required.
That means that the language is defective when we compare it with two other reasonably closely related languages such as French and German. Here's how it looks:

Person	Past verb forms			Future verb forms
Language	English	French	German	English	French	German
First singular	walked was/were walking	marchais	ging	will walk will be walking	marcherai	werde gehen
Second singular familiar		marchais	gingst		marcheras	wirst gehen
Second singular distanced		marchiez	gingen		marcherez	werden gehen
Third singular		marchait	ging		marchera	wird gehen
First plural		marchions	gingen		marcherons	werden gehen
Second plural familiar		marchiez	gingt		marcherez	werdet gehen
Second plural distanced		marchiez	gingen		marcherez	werden gehen
Third plural		marchaient	gingen		marcheront	werden gehen

You can see that in the past in French, there are five possible forms to choose from and in German, there are four. For the future, for which French can inflect the verb itself, we have a choice of six forms and in German, which also uses an auxiliary verb for the tense, the verb of choice gives us five forms to choose from.
Our choice depends on whether we are talking about first, second or third person forms, whether we are being polite and distanced or familiar and friendly or whether we are referring to plural or singular subjects.
English does none of that and is wholly defective in this respect, relying on a single form for all persons, all social relationships and all numbers.
To complicate matters further, French also has an aspect system and the verb forms above are just one of the possible selections because French also uses the verb have [avoir] to form a tense superficially similar to the English present perfect but which does not signal the same kind of relationship to time at all.
The past tense forms in French set out above qualify for translation into English as imperfective forms, akin to I was walking rather than I walked. The same form is used, incidentally, when English would employ the used to formulation for past habits. When French forms the tense with an auxiliary verb such as in:
J'ai marché
the translation would be I walked, not I have walked.
German, too, can form a tense in a similar way but that form, too, does not carry the same aspectual considerations as the English form does. It is quite arguable, in fact, that German does not have an aspectual tense system at all. The German tense formed with an auxiliary such as in:
Ich bin gegangen
can be translated either as I walked or I have walked.
Equally, as we see above, Standard German makes no distinction between imperfective progressive events and perfective simple ones.
To some extent, too, both French and German are defective because neither language distinguishes the verb forms for all numbers, persons and social relationships. English, however, is almost wholly defective.

The examples above are of other European Indo-European languages which are closely related (comparatively) to English. Other inflecting languages, such as the Slavic languages, other Romance languages like Spanish and Portuguese and Greek exhibit the same kinds of inflexions for person, social distance and number.
When we venture beyond this language grouping, things become even more complex and distinct.
If you find yourself teaching an inflecting language, considerable classroom time needs to be devoted to teaching and correcting the inflected forms of tenses.

The moral in all this is that proceeding in English to focus on form rather than meaning is not a sensible way to go. The tense forms themselves can be taught in minutes, even when we form aspects with the very irregular verb be.
The focus in teaching English has to be placed on meaning and for that to succeed, aspect is the key.

To do that, of course, requires you to become something of an expert on aspect in general and to be able to identify what the aspects in English do in terms of communication of notions and perspectives. For that to happen, some common confusions need dispelling.

	The confusion between progressive and continuous
I think they're talking nonsense

It is unhelpful to assume that continuous and progressive are simply alternative words for the same aspect (although many grammars do). We may, somewhat loosely, call the tense form something like present / past / perfect etc. continuous but when we consider aspect, we need to be a little more careful.
Nevertheless, it is true that in English little distinction in verb forms is made between continuous and progressive aspects so, for example:
John is working at his desk now
is progressive insofar as it refers to an action in progress, and
I am staying at The Ritz
is continuous insofar as it refers to a background state rather than an action currently in progress (I may be nowhere near The Ritz at the time of speaking).
As we shall shortly see, both these aspects can be realised through apparently simple forms of the verb, although the construction with the primary auxiliary be and the present participle is the most common choice.
It is important that teachers of English are aware that other people's languages may reserve explicit verb forms to distinguish between progressive and continuous aspects. Other languages, such as German, make no distinctions at all.

Continuous aspect: describes a current state of affairs such as, e.g.:
    I believe in fairies
    He loves me
    He is loving the attention
    He hated me
    We are on holiday
    She is living in a guest house
etc.
The clue is in the name – the aspect refers to the speaker's perception that something is a continuing state of affairs rather than a progressive or repeated action.
The continuous aspect does not always require the auxiliary + -ing. It can be inherent in both the simple form and the auxiliary + -ing forms of the verb. This is especially true, of course, with verbs such as think, believe, hate, understand, like etc. which contain the sense of the continuous semantically so do not exhibit any grammatical signalling of the continuous state.
Some of the examples above are simple tense forms but they nevertheless refer to a continuous state.
Progressive aspect: describes an ongoing action such as
    I'm typing this sentence
    He runs the length of the field and scores
    She is taking liberties
    We were eating in my favourite restaurant
etc.
The clue is in the name – the aspect refers to the speaker's perception that something is, was or will be in progress, i.e., an ongoing action.
Notice, again, that the progressive aspect does not always require the auxiliary + -ing. It can be inherent in both the simple form and the auxiliary + -ing forms of the verb. For example
    Look, I now turn over the paper and there is the shape
    Here she comes!
    I acknowledge receipt of your letter
all of which imply an ongoing action but none of which uses the auxiliary plus -ing form.
The simple form for a progressive action is, however, more rarely used than the simple form for a continuous state.

The -ing form of the verb can lead to some ambiguity because it can signal both continuous and progressive aspects of the verb. That is to say, it can signal a background event which is perceived as taking place in the present but is not necessarily something being done right this minute or a progressive, ongoing action happening now.
For example:
She is studying French at university
could imply that right this minute she is in a classroom or the library actively studying but probably refers to a continuous background event. She may not, at the time of speaking, be anywhere near the university in question. Compare this meaning with:
She's in the garden, pulling up weeds
which, because of the nature of the act and the prepositional phrase, almost certainly does refer to the present moment and is progressive rather than continuous.

The same form can also signal repeated events when it is used with a verb which cannot be conceived as progressive or continuous such as knock in:
Someone is knocking at the door
which is not progressive or continuous but iterative in meaning. See below under telicity for more on this.

	The confusion between stative and dynamic
I'm thinking about it

Much of the confusion evident in describing verbs as either stative or dynamic arises from the inability to distinguish between continuous and progressive aspects.

There is a distinction between, e.g.,
I am thinking
and
I think that ...
It is the distinction between the progressive aspect (I am thinking) and the continuous aspect with a simple verb form (I think). In the second of these, the verb is akin to believe (i.e., a continuous state of mind) and in the first, it describes an ongoing (that is to say progressing) action or process.

For this reason, if no other, it makes sense to speak of stative vs. dynamic uses of verbs.

Even verbs which do not refer to mental states may be used in both senses so, for the picture above, for example, we might say:
    She sat down on the bench (active use)
    She sat on the bench for a while (stative use)
    She suddenly sat on the bench (active use)
    She was sitting on the bench (stative use)
    She was just sitting on the bench when she remembered what time it was (active use)
    I saw her sit on the bench (active use)
    I saw her sitting on the bench (either use depending on the context)

Many other languages make no such distinctions between these uses of verbs and the expressions I go and I am going or I think and I am thinking are indistinguishable. English is not unique but it is slightly unusual. In particular, the use of progressive forms of tenses is often determined by a verb's stative or dynamic meaning (not its form). That is to say, the issue is semantic not grammatical.

Therefore, it is misleading and error-inducing to speak of stative or dynamic verbs (although many grammars use the terms as a shorthand). It does, on the other hand, make sense to consider the meaning of the verb in question and consider whether the use is stative or dynamic.

The confusions between perfective, imperfective and perfect

There are three concepts here:

Perfective: is the term used to indicate that an event or state is completed. For example,
    Napoleon died in 1821
is a perfective form which may or may not have present relevance but is clearly finished.
    I went to Margate last Thursday
is another example of a perfective form in English.
    I have given up smoking
is also a perfective form (because the action is completed) which happens to be in the perfect aspect as well, in this case because it has direct relevance to the present. We are making a crucial distinction here between perfective events and perfect aspects.
The perfective can be contrasted with the ...
Imperfective: which is the term that indicates an event is not completed. Examples are:
She was playing tennis with John
I have lived here all my life
In neither case is the event perceived as finished. The form of the verb is described as imperfect.
Many languages, the European Romance languages among them, distinguish perfective and imperfective notions by alterations to the form of the verb. The past tense forms in French, set out above, qualify for translation into English as imperfective forms, as was explained there.
Perfect: is the term used to signify an imperfective or perfective state or event which has a certain tense structure and carries a notion of the relativity of two events.
For example,
I have been to America
is a perfective (the act of going to America has been completed) but is a perfect tense form indicating that the past event is set in the present (and, therefore, has present relevance of some sort).
I have lived here all my life
is imperfective (but still a perfect form) because it also signifies some present relevance (in this case that the state is probably (not certainly) current).
The perfect is, in English grammar, contrasted with the simple.

Many other languages, again, make no such distinctions between these uses of verbs and the expressions I went and I have been are indistinguishable. English is not unique but it is slightly unusual.
Some languages are much clearer about perfective and imperfective forms. For more on other languages, see the guide to aspect.

Telicity: terminative, punctual and durative verbs

This is a related phenomenon in languages.
The term telic comes from the Greek, τέλος [telos], meaning end. The concept of telic and atelic verb phrases concerns whether an activity is seen as finished or unfinished. To grasp the point, compare:

I read the book for two hours
I read the book in two hours

Sentence 1. suggests that I did not finished the book so the verb phrase is atelic: no end is in sight.
Sentence 2. suggests that I finished the book so the verb phrase is telic: the end point is clear.

This matters in English because we can say:
I was reading the book for two hours
but we cannot have:
*I was reading the book in two hours.

The distinction here is between:

a time span adverbial such as
    for an hour
    throughout the week
    this year
which are all usable with progressive aspects of the verb so we allow:
    She had been running for a while
    They had been working throughout the weekend
    I have seen her this year
a time frame adverbial such as:
    in a minute
    within a week
    during the month
which are generally seen with a simple aspect of the verb so we allow:
    She will arrive in a day or so
    I will get it to you within a day or two
    He crashed during the race

The implication for English grammar is that verbs in the progressive aspect are almost always possible with a time span adverbial but not with a time frame adverbial.
A time frame refers to a period of time within which an action or event is set to finish but a time span is simply a period of time with no indication of an end to an event.
In other words, perfective forms, which are by definition telic, generally imply a time frame (even when it is not stated) but not a time span. For example:

I had been reading for an hour
is possible because for an hour refers to the time span in which the reading occurred, but
*I had been reading in an hour
is not possible because in an hour refers to a time frame
I was cycling for a week
is acceptable because the adverbial refers to a time span
but
*I was cycling in a week
is not allowed because the adverbial indicates a time frame
I have lived here for a year
is allowable because the adverbial indicates a time span
but
*I have lived here in a year
is not allowed because the adverbial indicates a time frame
I was reading when the telephone rang
is allowed because the telephone call occurs within a time frame but the reading is set within a time span
but
*I read when the telephone rang
is not allowed because the reading is set within a time span.

The situation is complicated by the fact that verbs are generally telic or atelic depending on their meaning so the phenomenon of telicity is as much semantic as grammatical. For example:

I finished the book in an hour
is telic by its meaning, because the adverbial refers to a time span, and
*I finished the book for an hour
is not possible because the adverbial refers to a time frame rather than a span.
I was cooking when the telephone rang
has the atelic verb cooking which is imperfective and the perfective rang which clearly had an end point so is telic. Compare:
I cursed when the telephone rang
in which both verbs are telic and perfective.
I strolled home in an hour
is telic because we can discern the end or goal of the activity but:
I strolled around the town in an hour
is not because stroll around is atelic in meaning so we need around for an hour.

If a diagram helps, it looks like this:
telic

Some languages, for example Finnish, Estonian, Czech and Hungarian, reserve a special verb form to signal telicity but English often does it through tense aspects with and without adverbials.

A semantic issue

In some languages (notably Russian) a distinction is made in the form of the verb between those which are terminative and those which are durative. English makes no such distinction in form but the concepts account for some of the stative / dynamic, perfective / imperfective and progressive / simple tense forms that can be used. The use of the term concepts here, of course, implies that we are talking about meaning not grammar directly. In other words the focus is semantic.

Terminative verbs suggest either of two closely related ideas
- a change of state which cannot, by its nature, be a progressive or currently continuing event, for example
      She struck the match
      They opened the box
      I sat down
- an action which cannot continue because it is punctual or momentary, for example
      The glass broke
      I wished her good morning
      They slammed the door
Durative verbs imply no such condition, for example
    She sang at the party
    I looked at the paper
    I was working in the office
Most verbs are, in fact durative, or can be understood that way, and include drink, drive, eat, enjoy, live, play, sing, sleep, stay, talk, wear, work and hundreds more. See below for a list of examples of non-durative verbs.

When terminative verbs which imply a change of state are used in progressive forms, they can be made durative so, for example, we can distinguish the sentences above as:
    She was peeling the potatoes
    They were opening the box
    I was sitting down
Terminative verbs which imply that an action cannot progress are less often used in this way and cannot usually be made durative so we do not usually encounter:
    *The glass was breaking
    *I was wishing her good morning
    *I was glancing at the clock
When terminative verbs are used in the progressive aspect, the implication is that the action was repeated. They are, in other terms, iterative so the last example strongly implies:
    I glanced at the clock repeatedly.

Durative verbs are commonly used in both the simple and progressive forms to imply continuous or progressive states. For example:
    I lived in London at that time (continuous)
    She stayed in a guest house on holiday (continuous)
    They were studying at university then (progressive or continuous, i.e., an action or a background state)
    He was running in the park (progressive)

Some terminative verbs can also be referred to as punctual or momentary verbs because their meanings cannot allow a durative interpretation. This is, therefore, a semantic not a grammatical distinction.
These verbs include those which refer to instant events such as arrive, bang, begin, break, bump, burst, chop, crash, detonate, dip, dive, drop, explode, flash, glow, hit, jolt, kick, light, meet, name, open, pop, quip, rap, shatter, shoot, slam, smash, spit, spurt, steal, stop, tap, thump, upset, volunteer, wake etc.
For example:
She is tapping on the window
cannot refer to a durative event because the verb itself is punctual or momentary. The sentence can only be interpreted as iterative in aspect because the action is being repeated.
One the other hand:
She is playing squash with her brother
is durative (and possibly prospective) and implies a progressive action rather than a momentary one.
Other examples include these pairs of sentences:

Progressive	Iterative
The light is glowing	The light is flashing
She was aiming	She was shooting
The dog was growling	The dog was barking
The rain is falling	The rain is splashing

Some verbs are interpretable either way depending on the co-text and context so, for example:
I am sitting on the train to London
almost certainly means that the action is durative and current (i.e., continuous) but:
I am taking the train to work these days
almost certainly means that the action is repeated (i.e., iterative) and the adverbial, these days, reinforces that sense.

Verbs such as steal also live on the borders between punctual and durative meanings so they can work as punctual and as durative verbs depending on the context. For example, we can have:
    He's stolen my pen
which is clearly a momentary event so
    He's been stealing my pen
makes little sense unless one believes that a pen can be stolen multiple times. Make it plural (pens) and it works:
    He's been stealing my pens
clearly relates to an iterated event.
On the other hand,
    Someone's been stealing vegetables from my garden
is a possible punctual verb used iteratively.
The verb work shares this characteristic. so, for example:
    I've been working too much recently
strongly implies a series of working days rather than a single event.
All the speaking verbs, say, speak, talk and tell, work this way, too.

	An example of telicity and punctual vs. durative verbs in action
knock, knock

To get this distinction really clear, you need to get the difference between the word continuous and the word continual. The first refers to an unbroken state so, for example:
The car makes a continuous growling noise
means that the noise is not interrupted but
The car makes a continual growling noise
means that the noise comes and goes.
So, continuous means constant and continual means often repeated.

We'll take the examples of the punctual verb, knock. and a durative verb wear. The first is a punctual verb because, by definition, the action is momentary and cannot be extended but it can be repeated. The second is durative because, by definition, the action continues for some time.
So, if we say, e.g.:
    I knocked
we are clearly using the verb telically because we perceive that the action is completed (so it's also perfective). Equally, if we say:
    I have knocked already
we know that this is a present tense message implying something like:
    So there's no need to knock again
or whatever and here, too, the meaning is perfective and telic because, by definition, the verb cannot be durative. The verb is also telic, of course, because by its nature the action had an obvious end point in time.
A durative verb such as wear sends a very different message because:
    I have worn this coat a lot
can be interpreted as meaning that I am wearing it now so is durative in meaning extending from the past into the present and the immediate future. In other words, the verb is atelic because no end point is recognisable to the hearer.
The time lines look like this:
telicity
in which the first one shows the current effect of the action and the second shows the durative nature of the verb wear. In both cases, the yellow line is deliberately extended into the immediate future as well as the present because this perfect tense in English refers to the present in relation to the past, not the other way round. That's why it's called the present perfect, of course.

Now, if we say:
I have been wearing this coat a lot
we are not, fundamentally, changing the meaning which is signalled. The time line will still be the same, to wit:
telicity2
and all we are doing is emphasising the duration of the verb. That's not all it does because the use of a progressive also makes the idea of the action continuing into the future more emphatic.

However, if we try the same trick with the verb knock, we cannot use the same time line because:
I have been knocking
must be represented with something like:
telicity3

in which it is clear that the use of a progressive form of a punctual verb results in an iterative aspect. It also implies quite strongly that the knocking will go on.
In other words:

Simple use of a punctual verb = telic and perfective.
Progressive use of a punctual verb = atelic, imperfective and iterative.
Simple and progressive uses of durative verbs = atelic and imperfective.

If we want to signal an iterative aspect of a durative verb, the simple form + an adverbial (such as again and again, repeatedly, frequently, repetitively, continually, time after time etc.) is the only choice so we allow, e.g.:
    I have worn this coat again and again
However, we cannot signal iterative uses of a durative verb using the progressive form because:
    *I have been wearing this coat again and again
is not allowed (in English) although:
    I have been wearing this coat all year
means continuously, not continually (i.e. constantly, not repeatedly) so is fine.
With punctual verbs the case is altered because both:
    I have been knocking again and again
    I have knocked again and again
are both allowed although the use of the adverbial in the first case is unnecessary and the second form is slightly odd because English speakers would usually keep it simple and select the progressive form without the adverbial to convey the message.

English, of course, does not distinguish these senses structurally (although it often uses adverbials, as we see, to make the sense clear) but other languages sometimes do change the form to distinguish the meanings. This is, of course a source of error for learners who may choose:
I was hitting it with a hammer
when no iterative sense is intended. This type of covert error is quite common.
The moral is that this matters in the classroom because if we try to teach tense forms without considering what sort of verb we are using, people will get confused and things wrong.
The distinction can be shows by presenting two different images like, e.g.:


He was hitting it with a hammer	He hit it with a hammer

In the guides which are linked from here to present and past time, some attention to durative and punctual verbs will be given, especially in the guide to talking about the past because that is where the distinction is often important.

There is also a guide on this site to lexicogrammar, which considers how the meaning of the lexis we use determines and sometime subverts the syntax (new tab).

Relative and absolute tense forms

Here, we depart slightly from a traditional description of tense forms in English and take a more functional view of what the forms actually do and the meanings they realise.

Absolute tense forms: These locate an event in time relative to the here and now, i.e., the time of speaking or writing. For example:
    She will take the 6 o'clock train
sets her action as lying in the future from now.
    She caught the 6 o'clock train
sets her action in the past seen from now
    She is getting on the 6 o'clock train
sets her action in the present or the present concerning a future arrangement
    She was sitting on the train
sets the continuous state in the past
    She will be sitting on the train
sets the continuous state in the future
Relative / relational tense forms: These take it a step further and relate what is happening relative to an absolute tense. For example:
    She has caught the train and is working on her laptop
sets the present (working) in relation to a past event (getting on the train). In other words, it embeds the past in the present. The present has been altered by a past event because the implication is that she could not be working on her laptop had she missed the train.
    She had caught the train and was working on her laptop
sets the past (working) in relation to the pre-past (getting on the train). In other words, it embeds the pre-past in the past.
    She will have finished by the time the train gets to London
sets the future (finishing) in relation to the post-future (getting to London). In other words, it embeds the future in the post-future.
Relational tense forms do not stop there. For example:
    She was sitting on the train, reading a book
is absolute in the sense that the events are fixed in time relative to the present as we saw above.
However,
    She was sitting on the train when she realised her mistake
sets the realising in relation to a background event and
    She had been going to work but had forgotten her laptop
sets the working as a future in the past (although one that did not happen).

An alternative way of seeing these forms is to consider what is being expressed in terms of time and the relationships between events and states, like this (following Lock, 1996: 149):

Time	Example	Explanation of the concept	Visualised green for present orange for past blue for future
The present in the present	He is working in the garden	The verb form refers to now set relative to now
The past in the present	He has worked in the garden	The verb form refers to pre-now in relation to now
The present in the past	He was working in the garden	The verb form refers to the present set in the past
The past in the past	He had worked in the garden	The verb form refers to the pre-past set in the past
The present in the future	He will be working in the garden	The verb form refers to the present set in the future
The past in the future	He will have worked in the garden	The verb form refers to the past set in the future
The future in the present	He is going to work in the garden	The verb form refers to the future set in the present
The future in the past	He was going to work in the garden	The verb form refers to the future set in the past

Seen this way, complex tenses which combine aspects can be explained conceptually.

The visualisation in colour on the right of these tables appeals to some learners. Do not present them all at once!

The page of example timelines to demonstrate some of the relationships set out here may help if you are considering teaching from this standpoint. Click here to go there (new tab).

Finite and non-finite forms

In these guides, we are mostly concerned with finite verbs forms (i.e., those marked in some way for person or tense). There is a guide to finite and non-finite forms on this site.
It should not be forgotten that non-finite forms can also signal aspect, even if they do not signal time or tense. For example:

Forms	Meaning	Aspects
Feeling rather shy, he put his hand up	At the time he felt shy and at the time he put up his hand: two processes, one continuous (the feeling) which is non-finite and one instant (the action) which is finite	continuous and simple
Being rather a tall man, he reached it easily	Two verbs, one a permanent state (tall) which is non-finite , the other an action (instantaneous) which is finite	continuous and simple
Arriving at station, he realised he was late	Two simple actions (arriving and realised), the first non-finite and the second finite	simple
If you will just hear me out	Two verbs (one apparently future but actually present and referring to willingness) and finite, the other an infinitive (non-finite)	simple

Moving on

If the distinctions above are clear to you, it's time to look at the ways English refers to time and the speakers' perceptions of time.

To check that you have taken this on board, try two tests.
If you would like a terminology matching task, there is one here.

Guides to times, tenses and aspects

In what follows this site takes a somewhat alternative view of tense grammar.

Wrong way round #1

Many grammars, especially those aimed at students, start with the name of the tense, explain its formation and then list the ways in which it is used and the meanings we make with the forms. It works along the lines of, e.g.:

The present progressive is formed by taking the appropriate present tense form of the verb to be, and following it with the -ing form of the verb (omitting a final 'e' if it is present) so we get, He is having lunch with his mother.
We use this tense to talk about current actions in progress, temporary states, the arranged future and ... etc.

That is familiar to most learners and teachers and may be reassuring. However, it is a non-functional way to proceed.
It is non-functional because it presumes a mental process something like this:
mental process 1

Here, we are going to work the other way: we will start with the meanings we want to make and identify the major ways to make these meanings using verb forms and their aspects.
This assumes a mental process more like:
mental process 2

In these guides, the assumption is that you know (and can teach) how the tenses and aspects are formed because, as we saw above, English has a very simple set of inflexions and frequently none at all.
We are only considering meaning and function.

Wrong way round #2

The second way in which what follows may be different from what you have read or seen elsewhere on the web is that we also try to get the thought processes the right way round when it comes to choosing adverbials to express aspectual meanings.
Here's an example, from the web, of getting things the wrong way around when referring to the uses of the simple past tense in English:

with for – to express the duration of something in the past
They lived in Hong Kong for 8 years.
We waited for 2 hours for our train.

The reason this is the wrong way around and misleading is threefold:

Only some verbs can be used in this sense of a durative aspect with the past simple form and those are, as was explained above durative verbs like live, stay, wait and so on which imply long-lasting events or actions
We cannot use this form with punctual verbs such as light, hit, open etc. because that gives us:
    We lit the fire then
    We hit the drum then
    She opened the parcel then
all of which imply a simple, short, finished action.
To make them signal a durative event, we are forced to use a progressive tense form as in:
    We were lighting the fire then
We can, however, form, for example:
    They lived in Paris then
and imply a durative aspect because the verb itself carries that sense and cannot refer to a simple, short, finished action.
This has nothing at all to do with the prepositional phrase with for. It may be appropriate in some senses to add for two hours / years / months etc. but that is not a determiner of the tense form to use. We do not start by selecting a prepositional phrase and then select a tense form to match it.
In any case, confining the form to the preposition for is both misleading and prescriptive and almost bound to lead to a teacher-induced error. There is a wide range of other adverbials which it may be appropriate (or not) to use with this aspect of the verb and they will include, e.g.:
    at that time
    when she was a child
    in the 1950s
and hundreds more.