When

from English Grammar Today

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 thewhen-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.