引导名词性从句的连接词可分为三类：
连词（5个）：that （本身无意义。引导单一的宾语从句时that常可省略，但引导主语从句、表语从句、同位语从句时that通常不被省略）
whether,if （均表示“是否”表明从句内容的不确定性）
as if ,as though （ 均表示“好像”，“似乎”）
◆以上在从句中均不充当任何成分
连接代词（10个）：what/ whatever, who/ whoever, whom/ whomever, whose/whosever, which/ whichever
连接副词（7个）：when/ whenever, where/ wherever, how/ however, 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 our team had won.
3. 在从句中做成分的连词. whether与if 均为是否的意思。 但在下列情况下，只可用whether：
1. whether引导主语从句并在句首。
eg:Whether he can come to the party on time depends on the traffic.
2. 引导表语从句。
eg:The question is whether we can get in touch with her.
3. whether从句作介词宾语。
eg:I am thinking about whether I should quit my present job.
4.if与whether都可以与or not 连用，但后面紧跟or not 时只能用whether。
We didn't know whether or not she was ready.（此时只能用whether）
I wonder whether/if the news is true or not.（此时则二者都可以用）
5.后面直接跟动词不定式时。
6.引导同位语从句时。
