when/wεn/

1.adverb

(interrogative) 什么时候

when did it happen?
什么时候发生的?
2.pronoun

the day when (relative) 当…的那一天

3.conjunction

(in time clauses) 当…时

when you've read it, tell me what you think
当你读过之后,告诉我你是怎么想的
be careful when you cross the road
过马路时要当心
that was when we needed you
那是我们需要你的时候
she was reading when I came in
当我进来时她正在阅读
I know when it happened
我知道什么时候发生的
4.conjunction

(whereas) 而

you said I was wrong when in fact I was right
你说我是错的而事实上我是对的
5.conjunction

(considering) 既然

why did you buy that when you can't afford it?
既然你买不起为什么还要买?