宾语从句的引导词
(1) 陈述句作宾语从句引导词用that。that在从句中不充当任何句子成分，无实际意思，在口语或非正式文体中常省略。如：
He said (that) it was cold in Moscow. Mother knows (that) Jim will work hard.
(2) 一般疑问句作宾语从句引导词用whether或if。whether或if表示是否、是不是的意思，不能省略。如：
I want to know if / whether the answer is right. Lucy asked if / whether they had a cheaper shirt.
(3) 特殊疑问句作宾语从句引导词用原来的疑问词，只不过在宾语从句中称作连接代词或连接副词。连接代词或连接副词在从句中充当一定的句子成分，有一定的意思，不能省略。如：
Please tell him which class you are in. He asked me whom I was waiting for.
Could you tell me where the post office is? Please ask the teacher how we can get there.