When

adverb
1.
at what time or period? how long ago? how soon?:
When are they to arrive? When did the Roman Empire exist?
2.
under what circumstances? upon what occasion?:
When is a letter of condolence in order? When did you ever see such a crowd?
conjunction
3.
at what time:
to know when to be silent.
4.
at the time or in the event that:
when we were young; when the noise stops.
5.
at any time; whenever:
He is impatient when he is kept waiting.
6.
upon or after which; and then:
We had just fallen asleep when the bell rang.
7.
while on the contrary; considering that; whereas:
Why are you here when you should be in school?
pronoun
8.
what time:
Till when is the store open?
9.
which time:
They left on Monday, since when we have heard nothing.
noun
10.
the time of anything:
the when and the where of an act.
1.
contraction of when is:
When’s the show over?
2.
contraction of when does:
When’s the next train leave?
3.
contraction of when has:
When’s he ever been an authority?
adverb
1.

at what time? over what period?: when is he due?
(used in indirect questions): ask him when he’s due

2.
say when, to state when an action is to be stopped or begun, as when someone is pouring a drink
3.
(subordinating) at a time at which; at the time at which; just as; after: I found it easily when I started to look seriously
4.
although: he drives when he might walk
5.
considering the fact that: how did you pass the exam when you’d not worked for it?
6.
at which (time); over which (period): an age when men were men
noun
7.
(usually pl) a question as to the time of some occurrence