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(1)表示不知道的原因时用because,即说话人认为听话人不知道,因此because从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虽然表示不知道的原因,但其语气较because要弱得多,是可说可不说的话,它只能置于主句之后,这时,for是并列连词.如果不是因果关系,而是对前面主句的内容加以解释或推断时,只能用for.如:It's morning now, for the birds are singing.(很显然,鸟叫不可能是"现在已是早上"的原因.)

(2)表示已经知道的原因时用as或since,即某种原因在说话人看来已经很明显,或已为听话人所熟悉,因此它是句中不很重要的部分.since要比as正式一些,它们通常被置于语句之前,但有时却相反.

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

(3)下列情况下只能使用because:

①在回答why的问句时;

②在用于强调句型时;

③被not所否定时.

Make a mark wherever you have any questions.

We will go where the Party directs us.

目的状语从句:that, so that, in order that

注意:目的状语从句的谓语动词常与情态动词连用,否则可能是结果状语从句.不可置于句首.

结果状语从句:that, so that, so … that, such … that …

注意:so + 形容词/副词 + that从句;such + 名词 + that从句.

方式状语从句:as, as if(though)

I'll do as I am told to.职称英语教材

It looks as if it is going to rain.

比较状语从句:than, as

条件状语从句:if, unless, so long as, in case, once, a far as, on condition that.

注意if与unless的区别:不能用and连接两个unless从句,即不能有… unless …,and unless … .

但if … not and if … not却不受此限.

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

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

注意状语从句中从句的省略现象

(1)连接词 + 过去分词

Don't speak until spoken to.

Pressure can be incrased when needed.

Unless repaired, the washing machine is no use.

(2)连词 + 现在分词

Look out while crossing the street.

(3)连词 + 形容词/其他

常见的有it necessary、if possible、when necessary、if any等.