1 quest You use when to ask questions about the time at which things happen.
When are you going home?..., When is the press conference?..., When were you in this house last?..., `I'll be there this afternoon.'—`When?'
2 conj If something happens when something else is happening, the two things are happening at the same time.
When eating a whole cooked fish, you should never turn it over to get at the flesh on the other side..., Mustard is grown in the field when weeds are there, rather than when the growing crops are there.
3 conj You use when to introduce a clause in which you mention something which happens at some point during an activity, event, or situation.
When I met the Gills, I had been gardening for nearly ten years.
4 conj You use when to introduce a clause where you mention the circumstances under which the event in the main clause happened or will happen.
When he brought Imelda her drink she gave him a genuine, sweet smile of thanks..., I'll start to think about it when I have to write my report.
5 conj You use when after certain words, especially verbs and adjectives, to introduce a clause where you mention the time at which something happens.
I asked him when he'd be back to pick me up..., I don't know when the decision was made...
6 pron You use when to introduce a clause which specifies or refers to the time at which something happens.
He could remember a time when he had worked like that himself..., In 1973, when he lived in Rome, his sixteen-year-old son was kidnapped.
7 conj You use when to introduce the reason for an opinion, comment, or question.
How can I love myself when I look like this?...
8 conj You use when in order to introduce a fact or comment which makes the other part of the sentence rather surprising or unlikely.
Our mothers sat us down to read and paint, when all we really wanted to do was to make a mess...