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(1)表示不知道的原因時用because,即說話人認為聽話人不知道,因此because從句是全句最重要的部分,通常它被置于主句之后.

You want to know why I'm leaving? I'm leaving because I'm full.

for雖然表示不知道的原因,但其語氣較because要弱得多,是可說可不說的話,它只能置于主句之后,這時,for是并列連詞.如果不是因果關系,而是對前面主句的內容加以解釋或推斷時,只能用for.如:It's morning now, for the birds are singing.(很顯然,鳥叫不可能是"現在已是早上"的原因.)

(2)表示已經知道的原因時用as或since,即某種原因在說話人看來已經很明顯,或已為聽話人所熟悉,因此它是句中不很重要的部分.since要比as正式一些,它們通常被置于語句之前,但有時卻相反.

Seeing all of the children already seated, he said,"Since everyone is here, let's start."

(3)下列情況下只能使用because:

①在回答why的問句時;

②在用于強調句型時;

③被not所否定時.

Make a mark wherever you have any questions.

We will go where the Party directs us.

目的狀語從句:that, so that, in order that

注意:目的狀語從句的謂語動詞常與情態(tài)動詞連用,否則可能是結果狀語從句.不可置于句首.

結果狀語從句:that, so that, so … that, such … that …

注意:so + 形容詞/副詞 + that從句;such + 名詞 + that從句.

方式狀語從句:as, as if(though)

I'll do as I am told to.職稱英語教材

It looks as if it is going to rain.

比較狀語從句:than, as

條件狀語從句:if, unless, so long as, in case, once, a far as, on condition that.

注意if與unless的區(qū)別:不能用and連接兩個unless從句,即不能有… unless …,and unless … .

但if … not and if … not卻不受此限.

You won't lose your weight unless you eat less and unless you exercise more.(×)

但可以說 … unless you eat less and exercise more.

注意狀語從句中從句的省略現象

(1)連接詞 + 過去分詞

Don't speak until spoken to.

Pressure can be incrased when needed.

Unless repaired, the washing machine is no use.

(2)連詞 + 現在分詞

Look out while crossing the street.

(3)連詞 + 形容詞/其他

常見的有it necessary、if possible、when necessary、if any等.