是用来讲明引起主句这个结果是什么原因的从句，在初中阶段引导缘由状语从句的从属连词主要的有because, as, since,for等。比如：
1.I didnt go to school yesterday because I was ill. 我昨天没去上学，由于我生病了。
2. Since everybody is h ere, lets begin our meeting. 既然大伙都来了, 让大家开始开会吧.
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,由于我有事要告诉她.
缘由状语从句应注意的问题
1、because , since , as , for,辨析
1)because语势最强，用来讲明人所不知是什么原因，回答why提出的问题。当缘由是显而易见的或已为大家所知，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引导的从句假如放在句末，且前面有逗 号，则可以用for来代替。但假如不是说明直接缘由，而是多种状况加以判断，就只可以用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有什么区别：一般情况下，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
汉语习惯上说由于所以，但英语习惯上却不可以将 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. 由于下雨，所以大家呆在家。
3、because 从句与 because of短语的转换
Because引导是什么原因状语从句有时可与because of 短语转换，because of后加是什么原因可以为好或坏 ，但最合法的还是because。
He cant come because he is ill. / He cant come because of his illness. 他因病不可以来。
I said nothing about it because his wife was there. / I said nothing about it because of his wifes being th ere. 由于他老婆在那儿，我对此事只字未提。
容易见到考法
对于缘由状语从句的考查，多以单选和完形填空的形式，从连词的意义角度让大伙选择连词。
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
分析： his eyesight was beginning to fail是he found it increasingly difficult to read是什么原因， 因此本题应使用可以表示缘由的连词for引导缘由状语从句。
答案： B
误区提醒
除去because , since , as , for这四个词的辨析外，对于because 从句与 because of短语的区别也是难题之一。Because引导是什么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
分析：题干的意思是 因为那场大雨，他没去上班 ，the heavy rain.表示缘由，排除C; since 和 because 表示缘由，后面跟的是从句，也就是说需要是一个完整的句子，本题中the heavy rain没主语和谓语，.只不过一个短语，所以排除A和C;because of表示因为，后跟名词、代词或短语。
答案：B
