when */*/*/

UK [wen] / US / US [hwen] adverb, conjunction

Summary:

When can be used in the following ways: - as a conjunction (connecting two clauses): When he saw me, he waved. - as a question adverb (introducing a direct or indirect question): When shall we meet? ♦ Do you know when these houses were built? - as a relative adverb (referring back to a noun and introducing a relative clause): I remember the day when the war began. (introducing a clause that is the subject, object, or complement of another clause): My proudest moment was when I received the poetry prize.

1) at what time or in what situation

a) used for asking at what time something happens

When will we know our test results?

When did the trouble start?

I asked him when he was going to start work.

"You'll get your money back." "Yes, but when?"

since when:

Since when have you been in charge of the project?

b) used for asking in what kind of situation something happens

When should I end a letter with "Yours sincerely"?

When is it OK to hug someone that you've just met?

c) used when someone knows or says at what time something happens, or in what situation it happens

He didn't say when they would leave.

It is important to know when you have had enough to drink.

2) at the time that something else happens

a) as soon as another action or event has finished

When it stops raining, I'll show you the garden.

b) at the same time as something else

I always wear a hat when I work in the garden.

when doing something:

She was often nervous when facing the camera.

c) at a point during the time that something else is happening

She claims she was at a friend's house when the shooting took place.

3) at a particular time or in a particular situation used for talking about a particular time or situation

The biggest surprise for Josie was when she was chosen as team captain.

the time/day/year etc when:

Do you remember the time when we took your mother camping?

There are some days when everything seems to go wrong.

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Usage note:
When can be left out or replaced by "that" when it follows a noun and introduces a relative clause: I remember the day (that) we first met.

4) used for introducing surprising information used for introducing a situation that makes someone's action or behaviour seem surprising

Why does she always drive to work when she could easily take the train?

He went on protesting his innocence when he was obviously guilty.

5) used for describing a general situation used for making a general statement about something that is always true or correct

We use "an" when the next word begins with a vowel.

Tonic water makes a refreshing drink when served with ice and lemon.