用作连接代词的what有三种典型用法：
1. 具有疑问性质，意为“什么，什么样的”。如：
I don't know what he needs. 我不知道他需要什么。(what在宾语从句中作宾语)
2. 具有感叹性质，意为“多么”。如：
Only then did I recognize what silly mistakes I had made. 只是在那时我才意识到我犯了多么愚蠢的错误。(what在宾语从句中作定语)
3. 具有关系代词性质的，意为“所…的”，此时相当于“the＋名词＋that”或者all that，即好像是既包括了先行词也包含了关系代词，因此，有学者称之为关系代词型what。如：
What (=The thing that) happened after that was interesting. 后来发生的事挺有意思。(what在主语从句中作主语)
China is not what (=the China that) used to be. (what在表语从句中作表语)
The boy dived into water and after what (the time that) seemed to be a long time, he came up again. 他跳进水里，过了好像很久之后，他才又从水里出来。(what在宾语从句中作主语)
