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高中英語語法主語從句與先行解析
高中英語語法主語從句與先行解析
主語從句與先行it
主語從句可以按其引導詞的不同分為三類：
第一類，用從屬連詞that引導的主語從句，例如：
That we shall be late is certain.
That the driver could not control his car was obvious.
這種結構主要是對that從句的內容進行強調，屬正式文體，連詞that不可以省略；但是在一般情況下，往往使用先行it結構，即用it作形式主語，而把that從句放到后面，這時，在口語中，連詞that有時則可以省略。所以上述兩句可以改為：
It is certain that we shall be late.
It was obvious that the driver could not control his car.
如果整個句子是疑問形式，就只能用先行it結構，例如：
Is it true that he would take the risk？
Is it possible that they will come tomorrow？
常使用先行it結構，用that從句作主語的句子有下列幾個句型：
1、It + be + 形容詞 + that從句：
It is clear that he was telling the truth.
It’s probable that we’ll be a little late.
2、It + be + 名詞詞組 + that從句：
It’s a pity that you can’t go with us.
3、It + 及物動詞 + 賓語 + that從句：
It worried her a bit that her hair was turning grey.
It shocked me that Peter didn’t tell anybody where he was.
4、It + be + 過去分詞 + that從句：
It is said that he has been there many times.
5、It + seem/happen/appear等不及物動詞 + that從句：
It seems that he has lost something.
注意：
在上述第1和第2兩種句型中，that從句前置與使用先行it，that從句后置在意義上沒有什么差異；但使用先行it結構較為常見。
在第3種句型中，that從句前置在語法上是可能的，但實際上并不常見，通常總是使用先行it結構 .
第4種句型實質上是被動結構，由于that從句不可以位于句首作被動句的主語，所以只能使用先行it結構。
第5種句型已經形成了固定的搭配關系，that從句不能前置，只能使用先行it結構，不過，這種結構可以轉換為帶有不定式的簡單句，例如：
It happened that I had no money with me that day.
→ I happened to have no money with me that day.
第二類，用連接代詞who、whose、which、what，連接副詞when、where、how、why，以及連詞whether（或if）引導的主語從句，例如：
Who was responsible for the accident is not yet clear.
→ It is not yet clear who was responsible for the accident.
What he did is not yet known.
→ It is not yet known what he did.
Whether it is true remains a problem.
→ It remains a problem whether / if it is true.
從以上例句可以看出，這類主語從句可以直接放在句首作主語，也可以使用先行it結構，把主語從句放在后面，兩種結構可以互換，意義上無差異，但用if 引導主語從句時，只能采取先行it結構，也就是說if不可以引導置于句首的主語從句。
第三類，用what、whatever、where、wherever、whoever、whichever等代詞引導的主語從句，例如：
What he said at the meeting encouraged everyone.
Whoever fails to see this will make a big blunder.
這類從句一般相當于帶有定語從句所修飾的名詞詞組，即在結構上相當于一個名詞加上一個定語從句，例如：
What he wants to buy is a ten-speed bicycle.
→ The thing that he wants to buy is a ten-speed bicycle.
Whoever breaks the law is sure to be punished.
→ Anyone who breaks the law is sure to be punished.
切記，這類主語從句不可以使用先行it結構。
上述第二類主語從句與第三類主語從句盡管在形式上十分相似，但實際上是不同的。第二類結構中用連接代/副詞引導的從句系由特殊疑問句轉化而來，以whether/if引導的從句系由一般疑問句轉化而來，自然含有疑問的意味，例如：
When they will have the sports meet is still a question.
→ When will they have the sports meet？
Who he is doesn’t concern me.
→ Who is he？
Whether he will join us won’t make too much difference.
→ Will he join us？
而第三類結構中的主語從句則沒有疑問的意味。
試比較下列各句：
① What caused the accident is a complete mystery.
② What caused the accident was a broken bottle.
③ What she looks like doesn’t matter.
④ What she’d like is a digital watch.
上述各句中盡管都有一個以what引導的主語從句，但其意義不盡相同；第①和③句中的主語從句系由What caused the accident？和What does she look like？轉化而來，所以可以改成：
It is a complete mystery what caused the accident.
It doesn’t matter what she looks like.
第②④句中的主語從句含義分別為The thing that caused the accident和The thing which she’d like，所以不含疑問意味，因此不可以說：
It was a broken bottle what caused the accident.
It is a digital watch what she’d like.
但是可以說：
It was a broken bottle that caused the accident.
It is a digital watch that she’d like.
不過，這已不是主語從句，而是強調結構了。
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