表语从句中引导词that和what有啥区别~

that是连词,不充当句子成分,what除了充当引导词还在从句中长当成分。如it was what they had been working so hard to find。what作find的宾语it is amazing that they have done this。此处that只做连词你要想好好学英语的话还是买本《英语常见问题解答大辞典》赵振才编的。还有本《初高中英语难点考点详解辞典》都蛮好的。