what引导主语从句也可，宾语从句也可,但不能引导定语从句。

what 引导主语从句的例子： What I am going to do depends on the weather. 我做什么取决于天气。这句话中what I am going to do 是整句话的主语从句，what 在主语从句中做do的宾语。宾语从句的例子： Tell him what he should do .其中what he should do 是整句话的宾语从句，what 在宾语从句中也做do的宾语。

可以做主语宾语表语……凡是名词能做的成分都能做

what you said surprised me做宾语

what surprised me surprised him too.作主语

what you feel can affect your decision.作表语