请赐教
what,which,是连接代词或关系代词，在引导从句时要充当从句主语宾语等成分。
that引导从句可无意义只代表后是从句，但that又可做关系代词引导定语从句，此时有意义用来指物指人。
I don't know what to do. what指做的事。
He told me that he was ill. that 引导宾语从句无实意，智世阁符号说明后是从句。
This is the book that I lost. 关系代词 that指the book 做从句宾语。