When as a question word
We can use when to ask for information about what time something happens:
When did you leave?
When are you going on holiday?
When will you know the result of the exam?
We can use when in indirect questions:
She asked me when I would be ready to start the job.
I wonder when the new computers will arrive.
When as a conjunction
We use when as a conjunction meaning ‘at the time that’. The clause with when is a subordinate clause (sc) and needs a main clause (mc) to complete its meaning. If the when-clause comes before the main clause, we use a comma.
Talking about the past
[SC]When I was young, [MC]there were no houses here.
[MC]Nobody spoke [SC]when she came into the room.
Talking about the present
When you start the engine, there’s a strange noise.
Talking about the future
In references to the future with when, we use the present simple or the present perfect in the when-clause, not the future with shall and will:
When the new park opens, I’ll go there every day.
Not: When the new park will open, I’ll go there every day.
When I’ve finished my homework, I’m going to phone Marita.
Not: When I’ll finish my homework, I’m going to phone Marita.
We can use when as a conjunction to mean ‘considering that’:
What’s the point in going out when we have to be home by eleven o’clock?
When as a relative pronoun
We can use when as a relative pronoun in relative clauses:
That was the week when we booked our holiday.
The parcel arrived in the post at 11 am, when I was still at work.
Since when?
We can use since when to ask at what time something began. We often use it as a response when we are surprised that something has begun:
A: Hilary’s working at the Art Museum now.
B: Really? Since when?
A: Oh, she’s been there about three months.
When: typical errors
Be careful not to use when instead of if:
If you arrive too late, you are not allowed to take the examination because they don’t accept late enrolment.
Not: When you arrive too late, …
Be careful not to use when instead of since:
I was very surprised to see him because it’s been a long time since I last saw him.
Not: … it’s been a long time when I last saw him.
We don’t use will after when to mean ‘at that time’:
When I start college, I’ll miss my old school friends.
Not: When I’ll start college, I’ll miss my old school friends.