是用来说明引起主句这个结果的原因的从句，在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 didn’t 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 here, let’s 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 didn’t 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 home. 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 can’t come because he is ill. / He can’t 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 wife’s being there. 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 found it increasingly difficult to read, ____ his 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 didn't go to work the heavy 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
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