when
used in questions
You use when to ask about the time that something happened or will happen.
When did you arrive?
‘They’re getting married.' – ‘When?’ – ‘Next month.’
used in time clauses
You use when in time clauses to say that something happened, happens, or will happen at a particular time.
He left school when he was sixteen.
When I have free time, I always spend it fishing.
If you are talking about the future, use the present simple in the time clause, not a future form. For example, say ‘Stop when you feel tired’. Don't say ‘Stop when you will feel tired’.
When you get to the hotel, go to reception and give your name.
I'll come when I finish work.
‘when’, ‘as’, and ‘while’
If you want to say what was happening at the time that an event occurred, you can begin by saying what was happening, then add a clause beginning with when.
I was just going out when there was a knock at the door.
We were at our desks working when we heard the explosion.
You can also use as or while to say what was happening when an event occurred. When you use one of these words, you describe the event in the main clause and say what was happening in the clause beginning with as or while.
As I was out walking one day, I saw a very unusual bird.
While I was standing at the bus stop, Raul came by.
If you want to say that two events are continuing to happen at the same time, you usually use while.
What were you thinking about while he was talking to you?
I don't like music playing while I am working.
used with ‘why’
When has another use which is not related to time. You can add a clause beginning with when to a question which begins with why, as a way of expressing surprise or disagreement. The when-clause shows the reason for your surprise or disagreement.
Why should I help him when he refused to help me?
Why worry her when there's nothing she can do about it?