Tense in dependent / subordinate clauses

I'll hang out the washing when the rain stops

English is slightly different from many languages insofar as the tense form in some subordinate clauses (or dependent – the terms are interchangeable) is not what one would expect. This leads to learner errors such as:
    *I'll hang out the washing when the rain will stop
    *I will tell him when I will see him
    *She will speak to the boss unless he won't have time
    *I don't care who will come
and so on.
Many languages happily allow, indeed demand, that the tense in the subordinate clause matches the time being spoken of. English frequently does not, so the correct version of the four examples is:
    I'll hang out the washing when the rain stops
    I will tell him when I see him
    She will speak to the boss unless he doesn't have time
    I don't care who comes
This is, naturally, non-intuitive for speakers of languages which are stricter about applying the right tense marker appropriate to the time reference.

Temporal dependent clauses

These are probably the most familiar. The subordinating conjunctions which most often link these clauses to the main clause are highlighted here:

I will talk to him after we have had lunch
He will arrive just as the party finishes
She will explain before the meeting finishes
Once we have the tickets we will be able to get in
I will wait till she arrives
They won't come until we have left
She will be here whenever we need her
They are going to arrive when the race starts
I will do that as soon as you want me to

In all these cases, the logic of the tense structure seen from the point of view of the speaker is undermined in English. Logically, the forms should be
    *I will talk to him after we will have had lunch (because both are relational future events)
    *She will explain before the meeting will finish (because both events are set in absolute time in the future)
    *She will be here whenever we will need her (because we are talking about two future states)
and so on.
In fact, some languages require this form of tense structure so, for learners from some language backgrounds, this is by no means easy.

Conditional dependent clauses

These are often taught as if they were a separate (and slightly mysterious) category but they fall into the same pattern as temporal clauses and can be taught together with them. They share grammatical characteristics.
The subordinating conjunctions which most often link these clauses to the main clause are highlighted here:

I will talk to him if we meet
He will arrive unless the train is late
She will pay as long as the meal isn't too expensive
Provided (that) we have the tickets we will be able to get in
Given that the meeting has finished I will be able to get home early
Assuming (that) the train is on time, we'll be with you around 10
She will be here presuming she has the time
They are going to come whatever their mother says
I will complain even if the boss is there

Again, the natural tendency in most languages is to use the logical tense in both clauses, leading to errors such as
    *I will talk if we will meet
    *Provided that we will have the tickets we will be able to get in
    *They are going to come whatever their mother will say
and so on.
The fact that time and tense are more firmly linked in many languages explains much about the difficulties learners encounter with English conditional structures.

Small odd ball

There is one conjunction which operates differently in AmE and BrE:
    She will take her laptop in case it's needed
is differently understood.
In British English, this means
    She will take her laptop and use it if she needs it
In American English, this might mean:
    She will take her laptop only if there's a need for it

Nominal dependent clauses

Dependent clauses in English also act as the object of verbs and in these cases, too, there is a tendency for English to use a present-tense form in the subordinate clause but the situation is slightly complicated by the fact that if the main clause refers to the present, the future is often used in the subordinate clause and vice versa.
For example:
I will want him to explain what he thinks about it
main clause refers to the future; subordinate clause is in the present
I want him to explain what he will do about it
main clause in the present; subordinate clause in the future
This, again, is non-intuitive for many learners whose languages do not allow this swapping around of tense forms.

Other nominal clauses produce similarly confusing tense forms especially after certain verbs to do with telling and thinking (verbal and projecting processes, technically). For example:
    Let us know how it turns out
    Tell us what you need
    It doesn't matter to me what he thinks
    I don't care who comes
and so on.
Naturally, this leads to error such as:
    *Let me know what you will want (which is only possible if the wanting is perceived in the future)
    *I don't mind who will come
etc.

The meaning of will

Elsewhere on this site, when analysing future forms, the anomalous nature of the modal will has been discussed. The verb is polysemous in that it can refer to a predicted future as in, e.g.:
    The forecast says it will rain
    I will / shall be thirty tomorrow
    The train will arrive at 6
etc.
but is also used to express current (i.e., present) volition as in, e.g.:
    I'll cook tonight
    I'll help you
    Will you come?
etc.
When used in its future-prediction meaning, the verb will can only appear once in the sentence, for example (and see above):
    He will come if you ask
not
    *He will come if you will ask).
However, when will clearly carries the volitional sense, the 'rule' is broken so we can have, e.g.:
    I will marry you if you will promise to give up drinking
compare
I will marry you if you promise to give up drinking
and
    I won't pay you if you won't finish the work
compare
    I won't pay you if you don't finish the work
In both those examples, the first modal refers to the future and the second modal refers to volition, not a predicted future, so will can appear twice in the sentence with different meanings.

Past-tense main clauses

When the main clause is in the past tense, all kinds of tense forms in the subordinate clause become possible. The rule is

If the main and subordinate clauses have different time references, English uses the tense that is relevant to the time of speaking.

Other languages, needless to say, have their own rules and conventions and they are usually different.

Examples will help:

I cooked the dinner because he was too busy
both main and subordinate clauses refer to the same time so the same tense is used for both
I cooked the dinner because he was going to be busy
past main clause relating to a future-in-the-past subordinate clause
I cooked so he won't have to
past main clause relating to a future seen from now
I cooked dinner so he has time to work
past main clause relating to a present condition
The dinner was better than he expected
past main clause and past subordinate clause relating to the same time
The dinner was better than he'll ever have again
past main clause relating to a future seen from now
The dinner was better than he thinks
past main clause relating to a present condition

The tendency in English to use a tense form relevant to the point of speaking can confuse learners and explains, incidentally, some mixed conditional forms. For example:
Providing he brought the car, we can get a lift to the hotel
where the bringing of the car is in the past but getting a lift is in the future.
If you haven't already told him, I won't mention it
where his not being informed is seen as a present condition although expressed in perfect aspect and the not mentioning is seen in the future.

	Subjunctive and putative should
I wish I were a better golfer
If this should go in, I win
If I hit it a little to the left it should go in

The subjunctive, to which there is a separate guide, is quite rare in English (but very common in many languages). The subjunctive occurs in, e.g.:

Present subjunctive:: The proposal is that the road be resurfaced
Whatever be your reason, it can't be allowed
I'll take my coat lest it rain
Past subjunctive:: What would the boss think if he were late?
I took a coat lest it rain
Suppose she were listening ...

The subjunctive, apart from in the semi-fixed, if I were you form, is mostly confined to formal contexts and even there it is, quite arguably, dying out and hardly worth teaching.
However, the language needs a replacement for it and this is where we find the putative form of the modal auxiliary verb, should.

All of the examples above can be rephrased using a putative form:
    The proposal is that the road should be resurfaced
    Whatever should be your reason, it can't be allowed
    I'll take a coat lest it should rain
    What would the boss think if he should be late?
    I took a coat lest it should rain
    Suppose she should have been listening ...

It is important to understand that should in this use does not carry its common sense of ought to. It is, somewhat formally, a replacement for the even more formal subjunctive. It is also not appearing here in its guise as the first-person equivalent of would.
Structurally, the form is not difficult but the question of style is more complex so it can be taught in set phrases quite readily. For example, all of these are semi-fixed expressions which can be learnt without any knowledge of the complexities:
    It's a pity that X should be Y (e.g., It's a pity that it should be so cold [formal])
    I'm surprised that X should Y (e.g., I'm surprised that he should threaten you [quite formal])
    It worries me that X should Y (e.g., It worries me that you should have stayed out so late [quite formal])
    How should I know? [informal]
    Why should X do Y? (e.g., Why should he be so insistent? [neutral]
and they are all putative uses of the verb but style varies very considerably.

The optative wish

Allied to the use of the subjunctive is the verb wish. This is one realisation of the optative function and is analysed elsewhere in the guide to suasion. The optative is directed at events or things that we cannot change so is often expressed using an unreal sense with would and the past tense of other modal auxiliary verbs.

The subordinate clauses can be connected with or without that.

Again, as we saw above, when the verb is used in a past-tense main clause, it is the speaker's temporal state which determines the tense in the dependent clause.
    I wish (that) it would rain (present tense of wish + past of will for futurity)
    I wish (that) she could be here (present tense of wish + past of can for ability)
    I wished (that) it would rain (past of wish + past of will for a future in the past)
    I wished (that) she could have been there (past of wish + perfect form of can for ability)

The verb is often followed by the subjunctive as in, e.g.:
    I wish (that) John were here
    I wish (that) she were nicer to him
This use is often, at least informally, replaced by a simple present as in:
    I wish (that) John was here
    I wish (that) she was nicer to him
but there are many who would judge this use as colloquial at best, wrong at worst.

So what?

For teaching purposes, the analysis of tense in subordinate clauses can be very fruitful, not least because it situates the feared conditional in relation to all sorts of other types of subordination which work in exactly the same ways. For example:

Once learners are familiar with temporal subordination such as
    I'll tell him when he comes
    I'm going to see him immediately after I have arrived
    He'll be busy so I'll help him with the work
and so on, then conditional subordination such as:
    I'll tell him if he comes
    I'm going to see him if I have time after work
    I'll help him if he's busy
etc. becomes much more accessible and less mysterious.
Combining the presentation can overcome much difficulty.
Making learners aware of the polysemous nature of the modal will is also fruitful because it explains apparently anomalous constructions such as:
    If he won't help we won't be able to finish in time
and also explains a great deal about the future forms in English once learners know that, e.g., B's statement in:
    A: There's no sugar!
    B: Oh, I'll get some. The shop will be open until 6.
contains two distinct uses of will. The first refers to B's current willingness (volition) and not to the future at all. The second use is concerned with the predicted future and is not to do with volition.
In conditional sentences, such as:
    If it rains, the garden will get watered.
contains the predictive rather than volitional use of will. Compare, e.g.:
    If there's no sugar, I'll get some
in which the word will refers to willingness, not futurity.
Getting away from the overly simplistic 'sequence of tenses' idea in, e.g., reported speech and result clauses allows learners to see the internal logic of tense use centred on the speaker. E.g.:
    He tells me he will be in London tomorrow
tomorrow refers to the speaker's temporal frame
    He won the lottery so will never work again
the speaker's future
    He won the lottery so didn't work again
the speaker's past
    He won the lottery so hasn't worked since
the speaker's present understanding.
Conditional forms in general lose much of their mystery. Once would is seen as the past of will, forms such as:
    I would help if you needed me
    He would help whenever he was asked
and the use of wish etc. become a good deal clearer.
Moreover, mixed conditionals and much else can only be properly understood when the idea of the time frame being centred on the speaker is grasped. For example:
    I wish I had brought the car but didn't so won't be able to take you
is bewildering unless the speaker's temporal viewpoint is considered.

Related guides
suasion	for more on the optative and some other related concepts
subjunctive	for a short guide to a little-used mood in English
future forms	for the guide to how English deals with the future
deixis	for more on where the centre is both physically and temporally
condition and concession	for more about conditionals and their relationship to other clause types

References:
Chalker, S, 1984, Current English Grammar, Basingstoke: Macmillan
Quirk, R, Greenbaum, S, Leech, G & Svartvik, J, 1972, A Grammar of Contemporary English, Harlow: Longman