Study this example:
Amy is on a train. She’s calling a friend.
‘I’ll call you again later when I arrive’ is a sentence with two parts:
the main part: I’ll call you again later
and when … : when I arrive
The time is future (‘later’), but Amy says:
…when I arrive (not when I will arrive)
We say
when I do something (not will do).
when something happens (not will happen)
Some more examples:
- We’ll go out when it stops raining. (not when it will stop)
- When you are here again, you must come and see us. (not When you will be)
- Don’t forget to lock the door when you go out. (not will go
The same thing happens aft er while / before / aft er / as soon as / until:
- What are you going to do while I’m away? (not while I will be)
- Before you go, there’s something I want to ask you.
- Wait here until I come back. or … till I come back.
You can also use the present perfect (have done) aft er when / aft er / until / as soon as:
- Can I have the newspaper when you’ve finished with it?
- Don’t say anything while Ian is here. Wait until he has gone.
We use the present perfect to show that one thing is complete before the other. The two things do not happen together:
- When I’ve phoned Kate, we can go out. (= first I’ll phone Kate and after that we can go out)
Do not use the present perfect if the two things happen together:
- When I phone Kate, I’ll ask her about the party. (not when I’ve phoned)
It is oft en possible to use either 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 or. You’ll feel better after you’ve had something to eat. something to eat.
if and when
Aft er if, we normally use the present (if I do / if I see etc.) for the future:
- I’ll be angry if it happens again. (not if it will happen)
- Hurry up! If we don’t hurry, we’ll be late.
We use if (not when) for things that will possibly happen (or not happen):
- 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 for them.
We use when for things which are sure to happen. Compare:
- I might go out later. (it’s possible) If I go out, I’ll get some bread.
- I’m going out later. (for sure) When I go out, I’ll get some bread.
