引导原因状语从句最常用的连词有because，for，as和since。
（1）because
1）语气最强，只有它才能用来回答why引导的问句。所引出的原因往往是听话人所不知道的或最感兴趣的原因。
例句
a. You want to know why I am leaving? I am leaving because I’m fed up with the job and the boss.
你想知道我为什么要辞职？我辞职是因为我厌倦了这份工作，受够了这里的老板。
b. My friends dislike me because I’m handsome and successful.
我的朋友都不喜欢我，因为我既帅气又事业有成。
2）可与表示强调的only，just连用。
例句
a. Just because I don’t complain, people think I’m satisfied.
b. You shouldn’t get angry just because some people speak ill of you.
（2）since
表示人们已知的事实、不需强调的原因。所以常译成“既然……”。since从句通常放在主句之前。
例句
a. Since Monday is Bob’s birthday, let’s throw him a party.
既然周一是鲍勃的生日，那我们为他办一场聚会吧。
b. Since everyone is here, let’s get started.
c. Since you are free tonight, why not drop in and play chess with me?
d. Since we don’t have class tomorrow, why not go out for a picnic?
e. Since you are sleepy, you might as well go to bed.
f.Since you are an English major, I guess you can help me with this sentence .
（3）as
as与since用法差不多，所引出的理由在说话人看来已经很明显，或已为听话人所熟悉，不需用because加以强调。
例句
a. She didn’t hear us come in as she was asleep.
b. I left a message as you weren’t there.
c. As Monday is a national holiday, all government offices will be closed.
d. I have to speak English with them as the guys I live with don’t know any Chinese.
（4）for
for表示推断的理由，是对前面分句的内容加以解释或说明。for所引出的分句通常放在主句之后。
例句
a. The days are short, for it is now December.
b. It rained last night, for the ground is wet this morning.
试比较：Because it rained last night, the ground is wet this morning.
c. It must be morning, for the birds are singing.
d. Pay attention to your enemies, for they are the first to discover your mistakes.