when,if,that,why,what的用法和区别答:连接词:that,whether,if (不充当从句的任何成分) 连接代词:what, whatever, who, whoever, whom, whose, which. 连接副词:when, where, how, why 不可省略的连词: 1. 介词后的连词 2. 引导主语从句和同位语从句的连词不可省略。That she was chosen made us very happy.We heard the news that ...