宾语从句即指在主从复合句中用作宾语的从句。宾语从句通常由连词that和whether (if)以及连接代词或连接副词引导。宾语从句的词序要用陈述句词序，不能用疑问句词序。
宾语从句的引导词
(1) 用that引导。that只起引导词作用，不充当句子成分，也没有词义，且通常可以省略：
We think (that) he will help us. 我们认为他会帮助我们。
He said (that) he was good at English. 他说他擅长英语。
I know (that) he is very busy. 我知道他很忙。
(2) 用if或whether引导。if和whether引导宾语从句，不充当句子成分，但有词义，表示“是否”：
I don't know if [whether] he will come. 我不知道他是否会来。
She asked if [whether] that was enough. 她问那是否够了。
I wonder whether [if] it is true. 我不知道是不是真的。
(3) 用连接代词引导。常见的引导宾语从句的连接代词有what, who, whom, which, whose等。此时要特别注意宾语从句用陈述句词序，而不用疑问句词序：
Ask him which he wants. 问他要哪一个。
She asked who would like to go. 她问谁愿意去。
Tell me whose wallet this is. 告诉我这是谁的钱包
【注】what引导宾语从句时，有时表示“什么”，有时表示“…所…的”。比较：
I asked what he needed. 你问他需要什么。
I gave him what he needed. 我给了他所需要的东西。
(4) 用连接副词引导。常见的引导宾语从句的连接副词有when, how, where, why等。此时要特别注意宾语从句用陈述句词序，而不用疑问句词序：
He knows where they live. 他知道他们住哪里。
Tell us how you are getting on now. 告诉我们你现在过得怎样？
I can't tell you why she is crying. 我不能告诉你她为什么哭。
He didn't say when he would return. 他没有说什么时候回来。