what 与that引导名词性从句时的区别啊?.

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.