同位語從句指的是在復合句中充當同位語的從句，可以對其前面的抽象名詞做解釋說明，屬于名詞性從句的范疇。同位語從句的引導詞為"that"，無含義，不充當成分，不可省略。

1. that引導同位語從句。

在下列名詞后可用that引導同位語從句answer, belief, doubt, fact, hope, idea, information, knowledge, law, news, opinion, plan, suggestion, thought, truth等。

I had no idea that you were here.

The story goes that William killed his wife.

He always works hard in spite of the fact that he is not in good health.

Suddenly the thought came to me that he would go blind.

注意：

①位語從句引導詞that無含義，不充當成分，不可省略。

②同位語從句與其說明的名詞或代詞為同一內容，故可以用is把前邊的中心詞和從句連接成一個句子。

③同位語從句前一般沒有逗號。

that引導同位語從句與引導定語從句的區別：that引導同位語從句時只起連接作用，不充當任何成分;that引導定語從句時，在從句中要作成分，如主語、賓語等。

①He told me the news that our team won.(that引導同位語從句)

②The news that / which he told me was very exciting.(that引導定語從句，在從句中作賓語)

2. 在no idea, question, problem等后可用wh-疑問詞引導同位語從句，if不引導同位語從句。

I have no idea where he has gone.

I have no idea what he said.

3. 辨析：引導名詞性從句時what與whatever, who與whoever的區別：

what 與whatever：

What you want has been sent here.(what = the thing(s) that/all that/anything that特指)

Whatever you want makes no difference to me.(whatever = anything that泛指)

What caused the accident was a broken bottle.(特指)

Whatever caused the accident has not yet been found.(泛指)

who 與whoever：

Who will go to Beijing on business has not been decided.(who表疑問，表示“誰”，“哪一個”)

Whoever breaks the law should be punished.(whoever = anyone who，表示“無論哪個人”)

4. 同位語從句注意與定語從句區別

同位語從句對前一名詞做補充說明，在從句中無語法位置，而定語從句所修飾詞在從句中占一語法位置如：

The idea that one can do the work without thinking is wrong.(同位idea在從句中無位置，而從句只是具體說明idea的內容)

The idea that you put forward at the meeting is wrong.(定語從句，idea做put forward 的賓語)