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When I do vs When I have done
Usually, when referring to the future, we use a time word or expression to define when something will happen.
- I’ll be there at 7:00.
- He’ll call you later.
Sometimes, however, if we want to give more detail, we can replace the time word with when/as soon as/until/once/before/after/while + subject + verb + etc. These are called future time clauses and they can go at the beginning or at the end of a sentence.
Future time clauses are similar to the first conditional. There’s a main clause and a when/after/etc. clause. We use the present (simple, continuous or perfect) in the when/after/etc. clause, and we normally use a future verb form in the main clause. But we CANNOT use a future form in the when/after/etc. clause.
- When I’m 70, I’ll retire.
- When I will be 70, I’ll retire.
- We’ll call you once we’ve arrived.
- We’ll call you once we will arrive.
Note that future time clauses generally follow the same structure as first conditional sentences; however, unlike conditional sentences, where we are not sure if something is going to happen, in future time clauses, we are certain that it is going to happen, so we replace the if with when/after/etc.
Present simple and present perfect in future time clauses
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We can use the present simple in a future time clause to refer to an action that will take place at the same time as the event in the main clause, and we can use the present simple or the present perfect when the action in the time clause needs to be completed before the future event in the main clause occurs.
This means that, very often, using the present simple or the present perfect makes little or no difference in the meaning of the sentence.
- I’ll move out when I find a job.
- I’ll move out when I’ve found a job.
- We won’t eat until the guests arrive.
- We won’t eat until the guests have arrived.
In the four sentences above, the events in the time clause need to be completed before the event in the main clause occurs, but using the present simple or the present perfect does not alter the meaning.
But sometimes, using the present simple or the present perfect in a future time clause can completely change the meaning of a sentence.
- We can discuss this when we have dinner. (=We can discuss this while we are having dinner.)
- We can discuss this when we’ve had dinner. (=We can discuss this after we have finished eating dinner.)
- When I cook dinner, I’ll watch the last episode. (=I’ll watch the last episode while I’m cooking dinner)
- When I’ve cooked dinner, I’ll watch the last episode. (=I’ll watch the last episode after I have finished cooking dinner)
When we want to stress that a future event can only happen once the action in the time clause has been completed, the present perfect is more suitable.
- You’ll have dessert when you’ve eaten all your peas.
- You will be able to get a pay rise once you’ve worked here for three months.
- I won’t let you play outside until you’ve done your homework.
Or sometimes, the present perfect should be used when the meaning of a sentence may be ambiguous. For example:
- I’ll call you when I have a shower. (It could mean during or after the shower, so it’s better to avoid the present simple.)
- I’ll call you after I have a shower.
- I’ll call you when I‘ve had a shower.
When the events in the time clause and the main clause happen together, we can use the present simple but NOT the present perfect.
- When I go to France, I’ll eat snails.
- When I’ve gone to France, I’ll eat snails.
- When I call Jack, I’ll invite him to the party.
- When I’ve called Jack, I’ll invite him to the party.
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