引导主语从句时，只能用whether，不能用if。

Question

引导主语从句时，只能用whether，不能用if。怎样才算是引导主语从句,请用最笨的方法说

Answer 1

主语从句(Subject Clause)

定义:如果一个句子在复合句中充当一个主语,那么这个句子就是主语从句.

第一部分：常规主语从句，即句子在复合句中充当一个主语

（1）That he finished writing the composition in such a short time surprised us all.

（2）Whether we will go for an outing tomorrow remains unknown.

（3）Who will be our monitor hasn't been decided yet.

（4） Whom we must study for is a question of great importance.

（5）What caused the accident remains unknown.

（6）Whatever you did is right.

（7）Whose watch was lost is unknown.

（8）What we need is time.

（9）What we need are good doctors.

小结:

（1)引导主语从句连词有that,whether,who,what,whatever等

（2)连词位于句首不能省略

（3)主语从句大多数情况下视为单三,但也有例外,如例9

第二部分：为了防止句子头重脚轻，通常把形式主语it放在主语位置，真正主语搁置于句末

（1）It is certain that he will win the match.

（2）It is true that he has made a very important discovery in chemistry.

（3）It is very likely that they will hold a meeting.

（4）It is strange that he should do that.

（5）It is important that we all should attend the meeting.

（6）It is strange that the man should have stuck to his silly ideas.

（7）It is a pity that we won't be able to go to the south to spend our summer vacation.

（8）It is still a mystery what caused the accident.

（9）It is said that he has gone to shanghai.(=He is said to have gone to shanghai)

（10）It is known to all that the gun powder was first invented by the Chinese.

（11）It is suggested that the work should be done with great care.

（12）It seems that he has seen the film.(=He seems to have seen the film)

（13）It happened that the two cheats were there. (=The two cheats happened to be there)

小结:

（1)以that 引出的主语从句,常以形式主语it引导. It is +形容词/名词/某些动词ed + that 从句.

（2) 在有些that从句中要用虚拟语气 (should+do/should+have done)例句4,5,6,11.

主语从句与宾语从句两者的用法和在句中的作用是不同的，下面从它们的各自用法，介绍：）~
一．主语从句
主语从句 定义:在句子中担当主语的是一个从句,这个从句就叫做主语从句。
主语从句是在复合句中充当主语的从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。
1. It 作形式主语和it引导强调句的比较
It 作形式主语代替主语从句，主要是为了平衡句子结构，主语从句的连接词没有变化。而it引导的强调句则是对句子某一部分进行强调，无论强调的是什么成分，都可用连词that。被强调部分指人是也可用who/whom。例如：
It is a pity that you didn’t go to see the film.
2. 用it 作形式主语的结构
(1) It is ＋名词＋从句
It is a fact that … 事实是…
It is an honor that …非常荣幸
It is common knowledge that …是常识
(2) it is ＋形容词＋从句
It is natural that… 很自然…
It is strange that… 奇怪的是…
(3) it is ＋不及物动词＋从句
It seems that… 似乎…
It happened that… 碰巧…
(4) it ＋过去分词＋从句
It is reported that… 据报道…
It has been proved that… 已证实…
3. 主语从句不可位于句首的五种情况
(1) if 引导的主语从句不可居于复合句句首。
(2) It is said , (reported) …结构中的主语从句不可提前。例如：
It is said that President Jingo will visit our school next week.
(3) It happens…, It occurs… 结构中的主语从句不可提前。例如：
It occurred to him that he failed in the examination.
(4) It doesn’t matter how/whether …结构中的主语从句不可提前。例如：
It doesn’t matter whether he is wrong or not.
(5) 含主语从句的复合句是疑问句时，主语从句不可提前。例如：
Is it likely that it will rain in the evening?
4. What 与that 在引导主语从句时的区别
What 引导主语从句时在句时在从句中充当句子成分，如主语．宾语．表语，而that 则不然。例如：
What you said yesterday is right.
二：宾语从句的几个特征：1、引导词：what\which\whose\when\whether\if\where
2语序：宾语从句必须是用陈述语句。
如：I think that you must work harder.
宾语从句的引导词、连接词的区别、否定转移等现象。
(1)表达时间的几个句型：一般用when或者是what time：
What time will the train leave?
由于时刻都是固定的，也可以用一般现在时代替一般将来时：
What time does the train leave?
(2)时间的表达方式：8：19(nineteen past eight),7:57(three to eight)，在时间的中间也可以加上分的单词(minutes)
(3)had better +动词原型。意思是提要求，建议。但是上下级不能用这种方式，因为语气太重：
You had better give up smoking .
(4)sb leave sth +地点
I left my book in my classroom yesterday.
(5)may I have your name(address\age)?一般询问对方的年龄、名字等，不要习惯以前的问法，这样很不礼貌，而是要采访上面的问法。以前的习惯是：how old are you? \what's your name?
(6)修饰名词的代词次序：限-描-大-形---年---颜----籍----物--------类+名词：
this is a bridge.
This is a beautiful bridge

参考资料：http://baike.baidu.com/view/100914.htm