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.