原因状语从句的从属连词because、as和for的选择，是大家经常会碰到的问题，尤其在做完形填空类的题目时，究竟该选择哪个，经常会困扰大家。不用担心，贴心的小编为大家整理了相当详细的解析：

在because，since，as这三个从属连词中，以because语势最强，since次之，as又次之。Because通常表示“新信息”，即表示一种未知的原因，而since / as则表示已知原因。所以，如前所述，在回答why-问句时要用because-分句，而不用for-分句，同样也不用since- / as-分句。作为分裂句中心成分，如前所述，也只能用because-分句之前可用否定词或其他修饰语，这种分句之前还可以用某些并列连词，但for-分句不可以，同样的since- / as-分句也不可以。关于这一点，我们再补充例子：

I went to the state university chiefly because the tuition was cheaper.

She didn’t want to go to Africa, probably because the weather is too hot.

I’m going there tomorrow because I have to, not because I want to.

由于since- / as-分句通常表示已知的原因，所以常常出现于句首，这又是它们与for-分句用法上不同之处。

例如：

As China is a long, narrow country, the temperature varies considerably from north to south.

Since he had a certain talent for composition, his English master encouraged him to write little pieces for the college magazine.

当然since- / as-分句也可位于句尾，作为一种补充说明。

例如：

I’ll have you ask someone else, since you can’t answer this question.

He saw her, as they were both getting off the bus at the same time.

看完了小编整理的语法解析，大家对打败because、as和for这三个“小怪物”是不是多了几分信心呢？赶快收藏起来吧~