宾语从句是一种名词性从句，在句中作及物动词的宾语，或介词的宾语，或形容词的宾语。根据引导宾语从句的不同连词，宾语从句可分为三类。
1. 由that引导的宾语从句。that只有语法作用，没有实在的意义，在口语和非正式文体中可以省略。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. 由连接代词who, whom, whose, what, which和连接副词when, where, why, how引导的宾语从句。这些连接代词和连接副词在宾语从句中充当某个成分。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引导的宾语从句。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.
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