When you get you r paper back, pay special attention to ( what) have been marked.为啥不用which

When you get you r paper back, pay special attention to ( what) have been marked.为啥不用which

首先，你想填which，是一个定语从句。定语从句就要有先行词。可以说成When you get your paper back, pay special attention to words/problems which have been marked.
如果要填一个词，就必须填what。此时，这个句子是一个名词性从句，即一个宾语从句。 pay special attention to 做谓语后面从句做宾语。此时需要用宾语从句引导词what引导。

When you get you r paper back, pay special attention to （pay attention to 是谓语，后面接的是一个宾语从句）( what) have been marked（What 可以引导一个名词性从句，这里作宾语，以为，做了标注的地方。而which放这里的话是个定语从句，必须在前面要有个先行词。）
谢谢~~~.

what=which parts

指paper 用which不好

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