that 一般引导的是定语从句和宾语从句 what 一般引导的是主语从句
what可以分解为something+that。例如：I don't understand what you said just now.本句的意思是 I don't understand the words that you said just now.
that一般跟在先行词后面。例如：Did you see the letter that came today?letter 在句中就是先行词。that在定语从句中作主语不可省略，但作宾语则可省略。例如：Did you get the book(that)I sent you? that在定语从句中作sent的直接宾语所以可以省略
一个是疑问代词，一个是指示代词
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