‘I’ll phone you when I get home’ is a sentence with two parts:
the main part: I’ll phone you
and the when-part: when I get home
The time in the sentence is future (tomorrow), but we use a present tense (I get) in the when-part of
We do not use will in the when-part of the sentence.
Some more examples:
We’ll go out when it stops raining. (not when it will stop)
0 When you are in London again, come and see us. (not When you will be)
0 (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:
0 What are you going to do while I’m away? (not while I will be)
0 I’ll probably go back home on Sunday. Before I go, I’d like to visit the museum.
0 Wait here until (or till) I come back.
You can also use the present perfect (have done) after when, after, until, as soon as:
0 Can I borrow that book when you’ve finished with it?
0 Don’t say anything while Alex is here. Wait until he has gone.
If you use the present perfect, one thing must be complete before the other (so the two things do not
0 When I’ve phoned Kate, we can have dinner.
( = First I’ll phone Kate and after that we can have dinner.)
Do not use the present perfect if the two things happen together:
0 When I phone Kate, I’ll ask her about the party. (not When I’ve phoned}
It is often possible to use either the present simple or the present perfect:
0 I’ll come as soon as I finish. or I’ll come as soon as I’ve finished.
0 You’ll feel better after you have or You’ll feel better after you’ve had
something to eat.
After if, we normally use the present simple (if I do I if I see etc.) for the future:
0 It’s raining hard. We’ll get wet if we go out. (not if we will go)
0 I’ll be angry if it happens again. (not if it will happen)
0 Hurry up! If we don’t hurry, we’ll be late.
When and if
We use when for things which are sure to happen:
0 I’m going out later. (for sure) When I go out, I’ll get some bread.
We use if (not when) for things that will possibly happen:
0 I might go out later. (it’s possible) If I go out, I’ll get some bread.
0 If it is raining this evening, I won’t go out. (not When it is raining)
0 Don’t worry if I’m late tonight. (not when I’m late)
0 If they don’t come soon, I’m not going to wait. (not When they don’t come)