which后加什么形式

which是疑问句的主语，后面应该用动词的单数第三人称形式；如果是定语从句的关系代词，需要看先行词的数。先行词是单数，谓语动词应该用单三形式；先行词是复数，谓语动词应该用动词原形。

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。

It was difficult for the inspectors to discover which documents were important and which were not.

检查员们难以发现哪些文件重要，哪些不重要。

Which planets can sustain life?

哪些行星可以维持生命的存在？

Which direction are you facing?

你面朝哪个方向？