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When

When is a wh-word. We use when to ask questions, as a conjunction and to introduce relative clauses.

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.
See also
  • Questions
  • Questions: *wh-*questions
  • Future

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?
See also
  • Conjunctions
  • Punctuation
  • As

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.
See also
  • Relative clauses

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.
warning

Using Since when at the start of a question can express anger or sarcasm:

  • [talking about children watching television]
  • A: Maybe you should try not to let them watch so much television.
  • B: Since when are you an expert on childcare? (This is very direct.)

When or if?​

We use when to refer to a future situation or condition that we are certain of, whereas we use if to introduce a possible or unreal situation.

Compare

When I see Gary, I’ll tell him that you said hello.I will definitely see Gary.
If I see Gary, I’ll tell him that you said hello.I may see Gary but I am not certain.
See also
  • If or when?

When or since?​

We use when to mean ‘(at) the time that’. We use since to refer to a particular time in the past until another time or until now:

  • I had a great time when I went to the coast.
  • I have been having a boring time since I came back home.
  • Not: I have been having a boring time when I came back home.
See also
  • Since

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.