是用来说明引起主句这个结果的原因的从句，在初中阶段引导原因状语从句的从属连词主要的有because, as, since,for等。例如：

1.I didnt go to school yesterday because I was ill. 　　我昨天没去上学，因为我生病了。

2. Since everybody is h ere, lets begin our meeting. 　　既然大家都来了, 让我们开始开会吧.

3. As you are in poor health, you should not stay up late. 　　既然你身体不好, 你就不该熬夜.

4.I asked her to stay to tea, for I had something to tell her. 我请她留下来喝茶,因为我有事要告诉她.

原因状语从句应注意的问题

1、because , since , as , for,辨析

1)because语势最强，用来说明人所不知的原因，回答why提出的问题。当原因是显而易见的或已为人们所知，就用as或 since。

I did nt go, because I was afraid.

Since /As the weather is so bad, we have to delay our journey.

2)由because引导的从句如果放在句末，且前面有逗 号，则可以用for来代替。但如果不是说明直接原因，而是多种情况加以推断，就只能用for。例：

He is absent today, becaus e / for he is ill. 　　He must be ill, for he is absent today.

3) as和for的区别：通常情况下，as引导的从句在主句前，for引导的从句在主句后。例： 　　As the weather is cold, I stay at home.(同义句) 　　I stay at home, for the weather is cold.

2、because习惯上不与so连用

汉语习惯上说因为所以，但英语习惯上却不能将 so与because 连用：

正：Because it was raining, we stayed at home. / It was raining, so we stayed at home.

误：Because it was raining, so we stayed at h ome. 因为下雨，所以我们呆在家里。

3、because 从句与 because of短语的转换

Because引导的原因状语从句有时可与because of 短语转换，because of后加的原因可以为好或坏 ，但最正规的还是because。

He cant come because he is ill. / He cant come because of his illness. 他因病不能来。

I said nothing about it because his wife was there. / I said nothing about it because of his wifes being th ere. 因为他妻子在那儿，我对此事只字未提。

常见考法

对于原因状语从句的考查，多以单选和完形填空的形式，从连词的意义角度让大家选择连词。

典型例题：He fou nd it increasin gly difficult to read, ____ hi s eyesight was beginning to fail.

A. and B. for C. but D. or

解析： his eyesight was beginning to fail是he found it increasingly difficult to read的原因， 因此本题应选用可以表示原因的连词for引导原因状语从句。

答案： B

误区提醒

除了because , since , as , for这四个词的辨析外，对于because 从句与 because of短语的区分也是难点之一。Because引导的原因状语从句有时可与because of 短语转换，because of后跟的是短 语 ，because后跟的是句子。

典型例题：He didnt go to work the heav y rain.

A .because B.because of C.so D.since

解析：题干的意思是 由于那场大雨，他没有去上班 ，the heavy rain.表示原因，排除C; since 和 because 表示原因，后面跟的是从句，也就是说必须是一个完整的句子，本题中the heavy rain没有主语和谓语，.只是一个短语，所以排除A和C;because of表示由于，后跟名词、代词或短语。

答案：B