一、当由陈述句充当宾语从句时，用that引导，that无词义，在口语或非正式文体中常省略。如：

　　She said that she would leave the message on the headmaster's desk.她说她会把留言条放在校长桌子上。

　　He said that he could finish his work before supper.他说他会在晚饭前完成工作

二、当由一般疑问句充当宾语从句时，用if或whether引导，意为“是否”。如：

　　Alice wanted to know if/whether her grandmother liked the bag.爱丽斯想知道她祖母是否喜欢这个包。

I don't know whether he'll come the day after tomorrow.我不知道他后天是否会来

但直接与or not连用时,往往用whether(if…or not也可以使用)。如：

　　Let me know whether / if he will come or not.(= Let me know whether or not he will come )让我知道他是否能来。

　　I don't know whether / if he does any washing¹ or not.(= I don't know whether or not he does any washing. )我不知道他在家洗不洗衣服。

I wonder whether we stay or whether we go.我想知道我们是去还是留。