as, because, since 与 for 这四个词均可用来说明一次行动、一种情况的原因，但它们之间有区别：

 (1) 在原因是一个句子的最重要部分时，一般用because，其从句也往往在句子后部。

• You want to know why I'm leaving? I'm leaving because I'm FED UP!
  你想知道我为什么要离开？我要离开是因为我受够了！
• People dislike me because I'm handsome and successful.
  人们不喜欢我是因为我人长得帅，事业上也很成功。

＊because 可以引出名词性从句作主语或表语：

• If you feel cold, it is because you didn't put on your overcoat.
  如果你感觉冷，那是因为你没穿大衣。
• Just because a child may grow up in the relative security of home and school and neighborhood doesn't mean that the human comedy-tragedy is not a part of daily life.
  就因为一个孩子可能在家庭、学校和社区相对比较安全的环境中长大，并不意味着人间的悲喜剧不是日常生活的一部分。

 (2) 当原因是已为人所知，或较句子其余部分次要时，用 as或since (since较as更正式一点)。as和since 从句往往放在句首。

• As women were not supposed to be novelists, she took the name of George Eliot. 由于女人被认为不应当小说家，所以她采用了乔治·艾略特这一名字。
• Since you are unwilling to co-operate, I'll have to find some other person to work with.
  既然你不愿合作，我将不得不另找别人一起干。

 (3) for表示的原因是作为事后想出的。for 所引出的从句对前面句子所说情况加以说明和解释，一般不放在句首。

• I decided to stop and have lunch — for I was feeling hungry.
  我决定停下来吃午饭——因为我感觉饿了。
• It must have rained last night, for the ground is wet.
  昨天夜里肯定下雨了，因为地是湿的。