引導名詞性從句的連接詞可分為三類：

連接詞：that,whether,if 不充當從句的任何成分)

連接代詞：what, whatever, who, whoever, whom,

whose, which.

連接副詞：when, where, how, why

不可省略的連詞：

1. 介詞后的連詞

2. 引導主語從句和同位語從句的連詞不可省略。

That she was chosen made us very happy.

We heard the news that our team had won.

比較：whether與if 均為"是否"的意思。 但在下列情況下，whether 不能被if 取代：

1. whether引導主語從句并在句首

2. 引導表語從句

3. whether從句作介詞賓語

4. 從句后有"or not"

Whether he will come is not clear.

大部分連接詞引導的主語從句都可以置于句末，用 it充當形式主語。

It is not important who will go.

It is still unknown which team will win the match.