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宾语从句中引导词的用法比较
在复合句中作主句的宾语，引导词有：
连词：that (that 常可省略),whether, if
代词：who, whose, what ,which
副词：when ,where, how, why 等。

（一）that引导的宾语从句（在非正式场合that可以省略）
1.可跟that从句做宾语的动词有：
say, think, insist, wish, hope, demand, imagine, wonder, know, suppose, see, believe, agree, admit, deny, expect, explain, order, command, feel, dream, suggest, hear, mean, notice, prefer, request, require, propose, declare, report等。
例句：The boy believes that he will travel through space to other planets.
注意事项：当主句谓语动词是 think, believe, suppose, expect 等词，而宾语从句的意思是否定时，常把否定转移至主句表示。
例句：I don’t think it is right for him to treat you like that.

2.在以下情况中that不能省略
a.当句中的动词后接多于两个由that引导的宾语从句时，第一个that可省，但后面的that不可省。
例句：He said (that) you were too young to understand the matter and that he was asked not to tell you.
b.当主句的谓语动词与that宾语从句之间有插入语时，that一般不可省。
例句：Just then I noticed, for the first time, that our master was wearing his fine green coat and his black silk cap.
c.当that从句是双宾语中的直接宾语时，that不可省。
例句：I can’t tell him that his mother died.
d.注:许多带复合宾语的句子，that引导的宾语从句经常移到句子后部，而用it作形式宾语。
例句：I find it necessary that we should do the homework on time.

（二）由whether,if 引导的宾语从句
1.由whether(if)引导的宾语从句，实际上是一般疑问句演变而来的。意思是“是否”。宾语从句要用陈述句语序。
一般说来，在宾语从句中whether与if可以互换使用，但在特殊情况下if与whether是不能互换的。
例句：I wonder whether(if) they will come to our party.

2.只能用whether，不能用if引导的宾语从句
a.在带to的不定式前
例句：We decided whether to walk there.
b.在介词的后面
例句：I’m thinking of whether we should go to see the film.
c.在动词后面的宾语从句时
例句：We discussed whether we had a sports meeting next week
d.直接与or not连用时
例句：I can’t say whether or not thet can come on time.

3.只能用if不能用whether引导的宾语从句
a.if引导条件状语从句，意为“如果”
例句：The students will go on a picnic if it is sunny.
b.if引导否定概念的宾语从句时
例句：He asked if I didn’t come to school yesterday.
c.引导状语从句even if（即使）和as if（好象）时
例句：He talks as if he has known all about it.