what不可以用来引导定语从句.

what可以用来引导主语从句、宾语从句、表语从句、同位语从句等。

例句：

1、What you did was quite right.

what引导主语从句。

2、I didn't hear what you said clearly.

what引导宾语从句。

3、This was what he had bought.

what引导表语从句。

4、I have no idea what he has done.

what引导同位语从句。