which从句的which后面是不是不能加助动词？

你是说which后面直接接上助动词么？还是说which引导的从句中出现助动词？
如果说是引导的从句的话基本上都可以出现助动词啊
比如说引导宾语从句I want to know which color he doesn't like.
主语从句 Which color he doesn't like is still unknown.
定于从句 This is the color which he doesn't like.
如果说是which直接加上助动词，那么which引导的从句中which充当主语就可以。
比如He is not going to by the shirt which doesn't look good on him.

比如说吧，We must give up the things which we don't need !在这个句子中which引导的就是一个定语从句，修饰和限定things，而need作为实意动词的时候，不能直接在后面加not，所以就需要用don't 这个助动词了。