原因状语从句的构成及引导词because,as,since,for等

状语从句 since 原因状语从句 as because 评论

原因状语从句多由because，as，since，for, seeing(that)，considering that, now that等连词引起：
You only do it because you have to．
你这样做只是因为不得不如此。
As he had been up since 4 am，he was very tired．
由于他清晨四点就起床了，他十分疲倦。
He can't come with us since he's sick．
由于生病他不能和我们一道去。
Stay in bed longer today，seeing that you were late last night．
昨夜你睡得晚，你今天多睡一会儿。
Considering he's only been learning English a year he speaks it very well．
考虑他学英语才一年，他英语是讲得很好的。
Now that you have the chance you had better avail yourself of it．
既然你现在有了机会，你最好加以利用。
I didn't go to school for I was sick.
我病了以没学校。

下面这类句子中的从句也表示原因：
I'm glad that you are all safe．
我很高兴你们都安然无恙。
I'm proud that you had the thought．
你有这个想法我感到骄傲。

because,as,since,for引导原因状语从句时的区别：
一般而，because表示直接原因，常用来回答why的提问；since表示显而易见或已知原因，语气较轻，常可译作“既然”；as表示已知原因；for表示推测原因，或附加，补充说明。
—— Why didn't you show up yesterday? —— Because I got sick.
—— 昨天你怎么没来？ —— 因为我病了。
Since you're sick, you'd better stay at home for a rest.
既然（因为）你病了，你最好在家里休息。
You must have been caught in the rain, for you're wet all over.
你一定淋雨了，因为你全身都湿透了。

注意：for引导的原因状语从句不能位于句首。