You use which after certain words, especially verbs and adjectives, to introduce a clause in which there is a choice between two or more alternatives.
In her panic she couldn't remember which was Mr Grainger's cabin.
慌乱中,她不记得哪间是格兰杰先生的船舱。
You use which to refer to a choice between two or more possible answers or alternatives.
I wanted to know which school it was you went to.
我想知道你就读的是哪所学校。
I can't remember which teachers I had.
我不记得哪些老师教过我。
You use which after a preposition to refer back to an idea or situation expressed in a previous sentence or sentences, especially when you want to give your opinion about it.
The chances are you haven't fully decided what you want from your career at the moment, in which case you're definitely not cut out to be a boss yet!
你也许目前还不能确定要从职业中得到什么,在这种情况下,你必然没有做老板的资质。
which is which
If you cannot tell the difference between two things, you can say that you do not know which is which.
They all look so alike to me that I'm never sure which is which.
它们在我看来简直一模一样,我永远分不清哪个是哪个。
You use which at the beginning of a relative clause when specifying the thing that you are talking about. In such clauses, which has the same meaning as that.
Soldiers opened fire on a car which failed to stop at an army checkpoint.
士兵们向那辆拒绝在军事检查站停车的汽车开了火。
You use which to refer back to an idea or situation expressed in a previous sentence or sentences, especially when you want to give your opinion about it.
They ran out of drink. Which actually didn't bother me because I wasn't drinking.
他们没有酒了,我倒觉得无所谓,因为我当时没在喝酒。
You use which in questions when there are two or more possible answers or alternatives.
'You go down that passageway over there.'—'Which one?'
“你顺着那边的过道走。” “哪个过道?”
Which vitamin supplements are good for you?
哪种维生素补品对你有好处?