1． 主語從句：主語從句在整個句子中作主語。
(1)that不同於其它的連詞，它不充當句子的成分，只是單純的連線詞，通常不可以省略。從句置於句首時，that絕對不可以省略。
that the driver could not control his car was obvious.
(2) if不能用在主語從句中，而是用whether
whether he left (or not) is unknown.
(3)當主語從句作主語時，謂語動詞一般用第三人稱單數，下面這個句型例外。
what we need is more time and money.
what we need are many more books.
(4)形式主語it代替後面的主語從句
it』s surprising that the little dog can work out math problems.
2.表語從句：表語從句出現在系動詞後，充當表語。
the trouble is that we are short of money.
(1) 引導表語從句除了上述三類詞外還有because, as if/ as though等等
the reason (why/for which…..) is that ……
it/this/that is because……
(2) 系動詞「appear, look, seem」的兩個常用句型
it seems/appears that….
it looks/seems as if/as though……(與事實相符用陳述語氣，與事實相反用虛擬語氣)
(3) as 也可以引導表語從句
things are not always as they seem to be
3. 賓語從句：及物動詞，形容詞和介詞後加賓語從句作其賓語。
(1) 賓語從句時態與主句相呼應，但賓語從句表示的是客觀真理或普遍現象除外
the teacher told us that the earth moves around the sun.
(2) 連詞whether和 if可以互換，但注意下列情況
連詞後緊跟or not時用whether
i want to know whether or not they will come.
作介詞賓語時用whether引導賓語從句
he was interested in whether the baby can stand by itself.
連線詞後直接加不定式，不能用if只能用whether
he doesn』t know whether to stay or not.
如果賓語從句是否定時，一般用if引導
i care if he will not attend the meeting.
(3) 當賓語從句後帶賓補時，要用「主語+謂語+it +賓補+that-clause, that不可省略
i think it certain that she will do well in her exam.
(4) 當主語是i, we，主句謂語是think, believe, suppose, expect, imagine等動詞時，用否定前移
i don』t think he will win the game, will he?
(5) that在賓語從句常可以省略，但由and或 but連線兩個或多個賓語從句時，僅可以省略第乙個連詞that
he said (that) he had eaten nothing but that he wasn』t hungry.
(6) 注意區別if引導的賓語從句和條件狀語從句
i don』t know if he will come.
if he comes, i』ll let you know.
4. 名詞性從句注意事項
1）疑問詞-ever既可以引導名詞性從句，也可以引導讓步狀語從句，而no matter+疑問詞只能引導讓步狀語從句
i believe whatever he says. （賓語從句）
whatever he says, i will never believe him. （讓步狀語從句）
no matter where i go, my heart is towards china.（讓步狀語從句）
×i believe no matter what he says. (錯句)
who與 whoever的區別
who是「誰」的意思，表示具有疑問，whoever相當於anyone who「無論誰」
whoever le**es the room last ought to turn off the lights.
can you tell me who that gentleman is?
2）．當名詞性從句缺少主語、賓語或表語時，引導詞通常用what，它相當於all that, anything that或 the thing(s) that。另外，有時具有感嘆意義
what we can』t get seems better than what we h**e.
tom thought what a beautiful girl mary was.
3）.注意區分it 作形式主語的主語從句與強調句
強調句句型it is/was+被強調部分+that, 去掉it is/was, that，句子仍然成立。
it is natural that they should h**e different views. 主語從句
it is only lately that he had had a family himself. 強調句
what was it that he wanted? i don』t know what it was that he wanted.
it was in the hotel where he stayed that we discussed the serious problem.
4）.注意it 作形式主語的主語從句和as引導非限制性定語從句引導的不同
it is known to all that the earth is round.
as is known to all, the earth is round.
常見考法
1.名詞性從句引導詞的確定；
作形式主語或賓語，代替後面的主語從句或賓語從句；
3.多個從句並存時的成分確定；
和if的異同等。
誤區提醒
1.引導詞的誤用；2. 對it作形式主語的句型掌握不牢固；3.從句種類確定錯誤導致相關知識錯誤；4.只用whether的情況記不清。
【典型例題】
1．____he does has nothing to do with me.
a. whatever b. no matter what c. that d. if
解析：錯選b。所選詞引導主語從句，而no matter what 不能引導名詞性從句。whatever引導主語從句並邏輯上作does的賓語。正確答案為a。
2. the manager came over and asked the customer how____
a. did the quarrel came about b .the quarrel had come about
c. h**e the quarrel come about d. had the quarrel come about
解析：錯選a或d。此題考查賓語從句的時態和語序。賓語從句引導詞在前，後面是陳述語序，故a、c、d都排除，正確答案為b。
3. energy is ____makes things work..
a. what b. something c. anything d. that
解析：錯選b。此題考查表語從句引導詞。
選something受漢譯影響。表語從句中缺少主語成分，要用what引導。如選something,後面還要加that.
正確答案為a。
4. ___is no possibility ____bob can win the first prize in the match.
a. there; that b. it; that c. there; whether d. it; whether
解析: 錯選b。此題首空不是形式主語，不用it.
因為不能說「bob比賽中能得第一不是可能性。」可能性要說「有/沒有」。故首空填there; 第二空填whether整句話意思不通，故正確答案為a。
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