when
[adverb]
Synonyms: then, again, anon
[conjunction]
Synonyms: though, albeit, although, howbeit, much as, whereas, while

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English Thesaurus: when, while, whenever, by the time, meanwhile, ...

[TahlilGaran] English Synonym Dictionary ▲

when S1 W1 /wen/ adverb, conjunction, pronoun
[Language: Old English; Origin: hwanne, hwenne]

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 air force 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.
GRAMMAR
Generally, in a clause beginning with when that refers to the future, use the present tense or present perfect, not 'will':
They don’t know what is going to happen when the project is finished (NOT when the project will be finished).
Remove the supports when the glue has set.
However, when asking or talking about the time that something will happen, use will:
When will the government take action?
teenagers who don’t let their parents know when they will be home

[TahlilGaran] Dictionary of Contemporary English ▲

when
pronoun
1.
BAD: When hearing that the child had been found, she burst into tears.
GOOD: On hearing that the child had been found, she burst into tears.

Usage Note:
To show that two things happen at the same time or that one thing happens immediately after the other, use on/upon doing sth (NOT when ): 'On examining the suitcase, he noticed that the locks had been tampered with.'

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2.
BAD: When I'll return home, I'll look for a better job.
GOOD: When I return home, I'll look for a better job.

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3.
BAD: I don't know when is she coming.
GOOD: I don't know when she is coming.

Usage Note:
See note at WHAT 3 (what)

[TahlilGaran] Dictionary of Common Errors ▲

when at or during the time that something happens:
Call me when you get home.
When I got here, the place was empty.
while during the time that something is happening – used to emphasize that something is continuing:
I’ll make the salad while you set the table.
While we were on holiday, our house was burgled.
whenever used in order to emphasize that something always happens when another thing happens:
He visits his mother whenever he can.
You can come and talk to me whenever you have a problem.
by the time used in order to say that one thing has or will have already happened when something else happens:
By the time a child is five, he will have watched hundreds of hours of television.

[TahlilGaran] English Thesaurus ▲

while during the time that you are doing something, or something is happening:
I bought a magazine while I was waiting for the train.
While we were on holiday, my bag was stolen.
meanwhile at the same time as something else is happening:
They’re still working on our bedroom. Meanwhile, we’re sleeping downstairs.
The Russians, meanwhile, declared war on August 8.
in the meantime during the period of time between now and a future event, or between two events in the past:
More aid is expected soon, but in the meantime these people are going hungry.
My new job hadn’t started, so in the meantime I tried to write a book.
whilst British English while. Whilst sounds a little more formal than while:
One American plane was forced to land whilst flying over North Korea.
Whilst all this was going on, the performers were getting ready for the show.
when at or during the time that something happens:
Call me when you get home.
When I got here, the place was empty.
whenever used in order to emphasize that something always happens when another thing happens:
He visits his mother whenever he can.
You can come and talk to me whenever you have a problem.
by the time used in order to say that one thing has or will have already happened when something else happens:
By the time a child is five, he will have watched hundreds of hours of television.

[TahlilGaran] English Thesaurus ▲