Longman Dictionary of Contemporary English homepage

Language: Old English
Origin: hwanne, hwenne

when

 adverb, conjunction, pronoun
     
when S1 W1
1 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.
2 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.
3 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.
4 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.
5 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.
6

by/since when

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.
7

since when ...?

spoken used to show that you are very surprised or angry:
Since when have you been interested in my feelings?
8 even though something is true:
Why does she steal things when she could easily afford to buy them?
9 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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