何时选择that和wh-做名词性从句的引导词

说说区别就行，还要特例

名词性从句缺成分时多用what
不缺成分时用that
比如:I know that you are clever. 宾语从句you are clever.是一个完整的句子，所以引导词就用that
I don't know where you live. 从句里单独用you live是不完整的，缺地点状语，所以就要用连接副词where了
当然只是举了宾语从句的例子，主语从句，表语从句，同位语从句都属于名词性从句，情况类同。

还有as

名词性从句有主语从句、宾语从句、同位语从句、表语从句。 that/whether引导同位语从句，主句并不缺少成分，从句也不缺少成分（区别于定语从句），that在从句中不做任何成分，从句起到补充说明的作用。可以跟同位语从句的名词通常有news，idea，fact，promise，question，doubt，thought，hope，message，suggestion，words（消息），possibility等（一般的“抽象”名词都可用）。eg: heard the news that our team had won．/He must answer the question whether he agrees to it or not. wh-引导主语从句、宾语从句、表语从句。主句缺少的成分用从句表示。eg：who is with long hair is David.(主语从句）we are looking at who is with long hair .(宾语从句)here is who is with long hair .(表语从句)