1
- at what time何时, 什么时候：
when did you last see him?
你上次见到他是什么时候?
with prep. since when have you been interested?你从什么时候开始感兴趣的?
1.1
- how soon多久：
when can I see you?
我何时能见到你?
1.2
- in what circumstances在何种情况下：
when would such a rule be justifiable?
在什么情况下, 这样的规定无可非议？
- at or on which (referring to a time or circumstance)当…时, 其时（指时间或情况）：
Saturday is the day when I get my hair done.
星期六是我做头发的日子。
1
- at or during the time that当…时, 在…时：
I loved maths when I was at school.
我在学校时喜欢数学。
1.1
- after然后：
call me when you've finished.
你完成后打电话给我。
1.2
- at any time that; whenever无论何时：
can you spare five minutes when it's convenient?
你能在方便的时候抽出5分钟的时间吗?
2
- after which; and just then (implying suddenness)一…就; 正在这时：
he had just drifted off to sleep when the phone rang.
他刚迷迷糊糊地睡下, 这时电话铃就响了。
3
- in view of the fact that; considering that既然; 考虑到：
why bother to paint it when you can photograph it with the same effect?
既然拍照也能达到同样的效果, 你干吗要费事去画它？
4
- although; whereas虽然; 然而, 可是：
I'm saying it now when I should have told you long ago.
我早就应该告诉你了, 可是我现在才说。
词源
Old English hwanne, hwenne; of Germanic origin; related to German wenn 'if'， wann 'when'.