Finite and non-finite verb forms

Definitions

The first thing to define is the term verb. In what follows, verb generally means verb phrase.
A verb phrase may be:

a single word: as in, e.g.,:
I went to London
a head verb with an auxiliary verb or verbs before it: as in, e.g.,:
I could have gone to London.
a verb or verb phrase modified by an adverbial: The adverbial can be an adverb, a noun phrase, a prepositional phrase or another clause, as in, e.g.,:
    I eventually managed it (adverb pre-modifier)
    I have finished this morning (noun phrase post-modifier)
    I studied at university in the 60s (prepositional phrases post-modifiers)
    I was happier when I worked alone (finite clause post-modifier)
This is not an analysis universally accepted. In some analyses, a verb phrase can only consist of verb forms, whether finite or not. For teaching purposes, because adverbials are so often embedded, it makes some sense to treat the whole item as a single unit.

What the difference between finite and non-finite verb phrases?

Finite verbs: are those which are clearly linked to definable subject and exist either in the present or past tenses. They show tense (for example, by changing the ending or the central vowel) and person (for example, by adding an -s for the 3rd person singular). They can also stand alone and retain their meaning.
Non-finite verbs: are not specifically tied to a subject and do not show tense or person. They can be:
adjectives:
    It's a smoking gun
    The window's broken
nouns:
    I like playing cards in the evening
    I hate waiting for buses
infinitives, with and without to:
    I came to help
    I want to complain
    She must help us

For example, the following contain finite and non-finite verb phrases, some modified, some not.

John went to see about the problem.
I very much enjoy sitting around doing nothing in the garden.
They spent all their money buying junk.
I am not near finishing the work.
She came hoping to talk to her sister
They would watch TV for hours on end

As a short exercise, comment on the verb phrases in the following, deciding if they are finite or non-finite.
Then click on the to reveal some comments.

She has stolen the money	This is a finite clause identifiable from the inflected form of the verb. It contains the non-finite -en form, stolen.
It’s tidy	This is a finite clause with an inflected form of the verb be.
Smoking is banned here	This is a non-finite verb operating as a noun.
I'm smoking too much	This is the same form of the verb but in a finite verb phrase with an inflected form of the verb be. It contains the non-finite smoking.
It made me sick	A finite verb phrase identifiable by the inflected past form of the verb make.
You must go now	A finite modal auxiliary verb with the non-finite bare infinitive.

In the third example, there's a finite form in the passive (is banned) and in the last example, there's a non-finite form: go (that's what the term infinitive implies, of course).

Confusion 1

Some confusion is caused in this area by the fact that verbs in English do not show very many inflexions. A verb may look the same but be performing different grammatical functions. In, e.g.
    I am playing
we have a finite verb phrase (with the non-finite -ing participle) but in
    I enjoy playing
we have a non-finite use of the verb play and a finite use of the verb enjoy which, in English, is not marked for tense or person.
Equally, in
    The game is played here
we have a passive finite verb phrase with the marked form of the verb be and the non-finite -en participle and in
    A much played game
we have a non-finite use of played as an adjective modified by the adverbial much.

Confusion 2

It is possible to describe individual verbs (rather than verb phrases) in the same terms. For example, the verb playing in I am playing is often described as a non-finite form (because it carries no marker for tense and person). However, in combination with the verb be, as in He is playing, the form is part of a finite verb phrase. We need to be careful with our terms and distinguish between a finite or non-finite clause and a finite or non-finite verb form.

Confusion 3

The -ing form and the -ed forms are both non-finite and often called present and past participles respectively. This is slightly misleading insofar as:

Regular verbs take -d or -ed depending on their morphology in both the past tense and the perfect forms so it becomes impossible simply by looking at the verb to say whether, e.g., played is the finite past tense of play or the non-finite past participle in, e.g., They have played.
The same confusion can arise with irregular verbs as many, such as catch, teach, read etc. have the same form for both past tenses and non-finite participles.
For this reason, many analyses distinguish them by calling the past tense form the -ed form and the past participle form the -en form. That is the system used above.
The -en form (or past participle) is not confined to past tense structures. It occurs for example, in
She will be invited
where it forms part of future passive form and
She is having her house redecorated
where it forms part of a present-tense causative structure.
Calling it the past participle may lead learners to conclude that it can only be used in past-tense structures.
The so-called present participle or -ing form exhibits similar problems because it can appear in non-present tense forms such as
She had been running to catch the bus
or
We will be spending our holidays in France next year
Again, calling it the present participle can mislead.
The -ing non-finite form has multiple functions. It can, for example,
1. form a gerund in:
  She dislikes swimming
2. form part of a progressive tense form in
  She is swimming
3. form part of an iterative (repeated) tense form in:
  She was singing in the choir during her teenage years
4. form part of a continuous tense form as in
  She was hoping for a pay rise
The -ing form is often said to be either a participle or a gerund but that, too, leads to confusion. The form can, in fact, occupy an intermediate position between a gerund proper (i.e., a verbal noun) and a participle (i.e., part of a finite clause). In, for example:
    The buildings were damaged
the noun buildings has been derived from the verb build but functions only as a noun (taking a plural form and not allowing an object as the verb does). Other derived forms work similarly, such as booking, carving, christening, clipping, covering, crossing, drawing, failing, flavouring, heading, meeting, mooring, offering, peeling, rambling, ruling, saving, setting, shaving, sighting, swelling, turning, warning etc. which can all be made plural and rarely take objects. Whether these should be described as gerunds or nouns is not clear.
However, in
    Playing the bagpipes is difficult
the word playing is slightly more verb-like insofar as it cannot take a plural (as countable nouns do) but may be treated as a mass noun although it clearly takes an object (the bagpipes) which nouns cannot do.
It may also appear with a possessive determiner as in:
    His playing was awful
where it is noun-like insofar as it forms the subject of the copular verb (was) with the complement attribute, awful. Compare, for example:
    His music was awful
which is parallel but clearly has a noun as the subject.
The form can, of course be fully verb-like in, e.g.:
    He was playing the bagpipes
when it forms part of a transitive verb phrase.
In between, we encounter some less obvious cases of noun-like and verb-like behaviours mixed and where the line is drawn is difficult to see. For example, in:
    I heard him playing the bagpipes
the reference is clearly to a progressive event (i.e., a finite tense form proper) and the -ing form takes an object. It is somewhere in between and could be classified as a gerund or a participle.

Verbs of perception

These verbs, see, hear, notice, observe, smell, sense, watch, feel work slightly differently with non-finite forms.
The key is to whether reference is to the whole action or a part of it, usually. For example:
    I saw him drink the beer
implies that I saw the whole action, whereas:
    I saw him drinking the beer
implies that I only saw a part of the action which started before and finished after my observation.
Equally, there is a difference between:
    I heard her sing at the concert
and
    I heard her singing upstairs
in which the first implies that I watched the whole performance and the second that I hear only part of her singing.
Compare, too:
    I saw him sit down vs. I saw him sitting in the restaurant
    I watched her repair the car vs. I watched her repairing the car
    I noticed him talking to the manager vs. I noticed him talk to the manager
    I felt the dog touch me vs. I felt the dog touching me
    I smelt the toast burn vs. I smelt the toast burning
and so on.
The difference stems from the sense of a progressive or background aspect with -ing forms of verbs in English and completed action with bare infinitives.

Finite verb forms

Finite verbs in English are sometimes identifiable by the changes to the verb form. Figure out which of the following English can show by changing the verb form and then click on the to reveal some comments.

Gender	English has no way to show this either in the singular or the plural. In some languages, the verb form will change depending on the gender of the subject (or the speaker)
Person	The -s inflexion in the present simple of a verb indicates the third person singular. This is the only person inflexion on regular verbs in English.
Number	The verb be distinguishes plural from singular (am vs. are, is vs. are) but the system is incomplete and most verbs only use the third-person -s to note singular vs. plural (he/she/it goes vs. they go).
Tense	This is indicated for most verbs by a change of form or the addition of a suffix (smoke-smoked, come-came etc.). Some common verbs, such as put and set, do not even show this change for tense.
Aspect	English uses auxiliaries to indicate aspect (the verb be for the progressive [she is arriving now] and the verb have for the perfect [she has arrived]).
Mood	English only omits subjects in the imperative (Go home!), sometimes uses an uninflected base form to indicate the subjunctive (If it be him) but does not otherwise indicate mood.
Voice	English has only two: active and passive. The voice is indicated by the use of the verb be, get or have (in causative structures).

As you can see, English finite verbs are barely inflected at all. Other languages do things very differently. In some languages, all of the above may be indicated by a change in the finite verb form and most inflected languages will show a greater range than English.

Non-finite verb forms

English has only three non-finite forms:

infinitives (with and without to):
he must go
I want to help
participles (past and present):
he has left
she is running
gerunds:
he dreads meeting her
overeating is a cause of illness

The distinction between a participle and a gerund is by no means as clear cut as this classification would imply as we saw above. It is probably better to consider a cline from purely participial use:
    He is meeting his wife at the airport
at one end and purely gerundial use
    That was an awful meeting
at the other, with less easily categorised forms in between
    I watched him running
and
    I objected to his running).
For more, see the article in response to a visitor's question.

However, non-finite forms appear all over the place in different guises. What are they in the following?
Decide and then click on the to reveal the answers.

Running is tiring	Gerund as the subject
I hate running	Gerund as the object
He is good at running	Gerund as the complement of a preposition
I want to go	Infinitive with to after a main verb
He let me go	Infinitive without to after a main verb
To go would be foolish	Infinitive with to as a subject
There's no call to go	Infinitive with to post modifying a noun phrase
I am going	Present participle
I have gone	Past participle
I was forbidden to run	Passive participle + infinitive with to

The last example shows something called a verb chain (they are catenative verbs to which there is a guide on this site linked in the list of related guides at the end). There are two in this case, as is usual. For example,
    They agreed to try to come
    The hoped to persuade her to come
    They remembered asking me to help
where the non-finite forms have no dependent subjects.

Non-finite verb forms as subordinators or prepositions

One important function of non-finite verb forms in English is to subordinate one clause to another. For example, instead of
    When he opened the bonnet he saw the problem (using when as a subordinating temporal conjunction)
we can have:
    On opening the bonnet, he saw the problem
and instead of:
    If you say that, he'll be furious (a subordinating conditional using if)
we can have
    Saying that will make him furious
Learners, incidentally, do not invariably recognise the in-built conditionality that many non-finite clauses contain because some languages simply cannot do that.
Cause and effect are often signalled by non-finite forms following prepositions as in, for example:
    To do it, he worked all night
    As a result of working all night, he got it done
    He went in order to see her

Some non-finite forms have become established as conjunctions and occur frequently in that role. For example:
    Providing that you pay the rent, she can't evict you
    Granted that we can find the money, I see no problem
    Supposing it rains?
    Provided only that we have time, we'll see you on Friday

Others may take on the nature of prepositions as in, for example:
Including Mary, we shall need six tickets
I see no problem regarding the timescale

There is a guide to subordination on this site.

Comparing languages

English is extremely concise in some ways. For example, the -s ending on She works indicates:

person (third)
number (singular)
tense (present)
aspect (simple)
mood (indicative)
voice (active)

However, at other times the language seems clumsy, ambiguous and inefficient. For example,
She might have been told
contains three auxiliaries (and a non-finite form) which separately indicate:

modality (might)
perfect aspect (have)
passive voice (been)

To make matters worse, some of these auxiliaries indicate different things at different times. For example, in
They will have been working
the auxiliary been now indicates progressive aspect and not passive voice. That can be deeply confusing for learners of the language, especially those whose first languages have different ways to signal progressive forms (if they do so at all) and the passive.

In other languages, such as Greek or Russian, most or all of these can be expressed in a single verb form (as English did in the example She works).

Related guides
conjunction	for more on how clauses are connected and links to other guides to subordination and coordination
subordination	for a guide which contains consideration of how the use of non-finite forms is restricted to certain subordinators
clauses	for more on clause structures
verb and clause types	for a guide to the six main sentence structures in English
phrases	for a general guide to phrase structures
infinitive	for the analysis of one sort of non-finite form
catenative verbs	for a guide to important uses of non-finite forms
nominal clauses	for an analysis of the ways finite and non-finite clauses can act as noun phrases

Take a test on some of this.