賓語從句是一種名詞性從句，在句中作及物動詞的賓語，或介詞的.賓語，或形容詞的賓語。根據引導賓語從句的不同連詞，賓語從句可分為三類。
1. 由that引導的賓語從句。that只有語法作用，沒有實在的意義，在口語和非正式文體中可以省略。例如：
He said （that） he wanted to stay at home.
She doesn't know （that）she is seriously ill.
I am sure （that） he will succeed.
2. 由連線代詞who, whom, whose, what, which和連線副詞when, where, why, how引導的賓語從句。這些連線代詞和連線副詞在賓語從句中充當某個成分。例如：
Do you know who（whom）they are waiting for?
He asked whose handwriting was the best.
Can you tell me where the No.3 bus stop is?
I don't know why the train is late.
3. 由if或whether引導的賓語從句。If和whether在句中的意思是是否。例如：
I want to know if（whether）he lives there.
He asked me whether（if）I could help him.
