Study this example:
A: What time will you call me tonight?
B: I’ll call you when I get home from work.
“I’ll call you when I get home from work” is a sentence with two parts: “I’ll call you” (the main part) and “when I get home from work” (the when part). The sen tence is future (tonight), but you cannot use will or going to in the when part of the sentence. Instead we use a present tense, usually simple present (I do).
•I can’t talk to you now. I’ll talk to you later when I have more time. (not when I’ll have)
• When the rain stops, we’ll go out. (not when the rain will stop)
The same thing happens after:
while after before until/till as soon as
•Can you take care of the children while I am out? (not will be)
• Before you leave, you must visit the museum. (not will leave)
• Wait here untill I come back. (not will come)
You can also use the present perfect (I have done) after when/after/until, etc., to shos that the first action will be finished before the second:
•After I’ve read this hook, you can have it.
• Don’t say anything while Tom is here. Wait until he has gone.
It is often possible to use either the simple present 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 when you have something to eat. or You’ll feel better when you’ve had something to eat.
After if we also use the simple present (I do) for the future:
• It’s raining. We’ll get wet if we go out. (not if we will go)
• Hurry up! If we don’t hurry, we’ll he late. (not if we won’t hurry)
Be careful not to confuse when and if. Use when for things that are sure to happen:
I’m going shopping this afternoon. When I go shopping, I’ll buy some food.