the last sentence which don't understand有什么错？

从句没主语，可以改成：
the last sentence which I don't understand
或是：
the last sentence which is not understood.

the last sentence which I don't understand
那个我不理解的最后一个句子。

貌似没有主语

WHICH 后面没东西啊，是谁不理解啊，OH MY GOD