A. Study these examples:
A: What time will you phone me tomorrow?
B: I'll phone you when I get home from work.
'I'll phone you when I get home from work' is a sentence with two parts:
the main part: 'I'll phone you' and the when-part: 'when I get home from
work (tomorrow)'
The time in the sentence is future ('tomorrow') but we use a present tense
(get) in the when part of the sentence.
We do not use will in the when-part of the sentence:
Examples:
* We'll go out when it stops raining. (not 'when it will stop')
* When you are in London again, you must come and see us. (not 'when you will
be')
* (said to a child) What do you want to be when you grow up? (not 'will grow')
The same thing happens after: while/before/after/as soon as/until or till
Examples:
* I'm going to read a lot of books while I'm on holiday. (not 'while I will be')
* I'm going back home on Sunday. Before I go, I'd like to visit the museum.
* Wait here until (or till) I come back.
B. You can also use the present perfect (have done) after when/after/until/as
soon as:
* Can I borrow that book when you've finished it?
* Don't say anything while Ian is here. Wait until he has gone.
It is often possible to use the present simple or the present perfect:
* I'll come as soon as I finish. or I'll come as soon as I've finished.
* You'll feel better after you have something to eat. or You'll feel better after
you've had something to eat.
But do not use the present perfect if two things happen together. The present
perfect shows that one thing will be complete before the other (so the two things
do not happen together).
Compare:
* When I've phoned Kate, we can have dinner. (= First I'll phone Kate and after
that we can have dinner.)
but * When I phone Kate this evening, I'll invite her to the party. (not 'when I've
phoned') (In this example, the two things happen together.)
C. After if, we normally use the present simple (if I do/if I see etc.) for the future:
* It's raining hard. We'll get wet if we go out. (not 'if we will go')
* Hurry up! If we don't hurry, we'll be late.
Compare when and if:
We use when for things which are sure to happen:
* I'm going shopping this afternoon. (for sure) When I go shopping, I'll buy some
food.
We use if (not 'when') for things that will possibly happen:
* I might go shopping this afternoon. (it's possible) If I go shopping, I'll buy some
food.
* If it is raining this evening, I won't go out. (not 'when it is raining')
* Don't worry if I'm late tonight. (not 'when I'm late')
* If they don't come soon, I'm not going to wait. (not 'when they don't come')
