How to use
at what time
When are we leaving?
When did you first meet Dr Darnall?
When will the work be finished?
I don't know when I'll see her again.
when to do something
I'll tell you when to stop.
at or during the time that something happens
Leonard was nine when his father died.
When the family came here from Russia, they were penniless.
When he was in the airforce he flew Tornado jets.
after or as soon as something happens
When the meal was finished, Rachel washed up and made coffee.
I'll phone you again when I get home.
used to mention a type of event or situation when talking about what happens on occasions of that type
When lead is added to petrol, it improves the car's performance.
When mixed with water the powder forms a smooth paste.
He always wears glasses except when playing football.
used to show which particular time or occasion you are talking about
The best moment was when he scored the winning goal.
There are times when I hate him.
the day/time/afternoon etc when
She remembered the day when Paula had first arrived.
before or since which time
The baby is due in May, by when the new house should be finished.
That was written in 1946, since when the education system has undergone great changes.
since when ...?
used to show that you are very surprised or angry
Since when have you been interested in my feelings?
even though something is true
Why does she steal things when she could easily afford to buy them?
used to introduce a fact or statement that makes something seem surprising
When you consider that the airline handled 80 million passengers last year, the accident figures are really very small.
Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.
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