Murphy. Unit 9. When and If sentences. When I do …or If I do…
Study this example:
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)
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 sentence 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).
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 until I come back, (not will come)
After I’ve read this book, 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:
You can also use the present perfect (I have done) after when / after / until, etc., to show that the first action will be finished before the second:
I’ll come as soon as I finish.
You’ll feel better when you have something to eat.
I’ll come as soon as I’ve finished.
You’ll feel better when you’ve had something to eat.
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 be late, (not if we won’t hurry)
After if we also use the simple present (I do) for the future:
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.
Use if (not when) for-things that will possibly happen:
I might go shopping this afternoon. If I go shopping, I’ll buy some food.
If it rains this evening, I won’t go out. (not when it rains)
Don’t worry if I’m late tonight. (not when I’m late)
If he doesn’t come soon, I’m not going to wait, (not when he doesn’t come)