1. because引导的原因状语从句一般放于主句之后， because表示直接原因， 语气最强， 最适合回答 why引导的疑问句。例如： I do it because I like it. 我做这件事是因为我喜欢。
注意： “not ... because”结构中的not否定的是because引导的整个从句， 例如： The country is not strong because it is large. 国强不在大。
2. since引导的原因状语从句一般放于主句之前表示已知的、 显然的理由(通常被翻译成“既然”)， 较为正式， 语气比because弱。例如： Since you are free today, you had better help me with my mathematics. 既然今天你休息， 你帮我补习数学。
注意： seeing (that)， now (that)， considering (that)， in that这几个词汇与since引导的原因状语从句意思相近， 都表示“既然”。例如： Seeing (that) he refused to help us, there’s no reason that we should now help him. 他既然曾经拒绝帮助我们， 我们现在没有理由要来帮助他。// Now (that) you are grown up, you should not rely on your parents. 既然你长大了， 就不应该依靠你的父母了。// Considering (that) everybody is here, let’s begin our discussion. 既然大家都到了， 我们就开始讨论吧。// In that he is ill, he feels unable to do it. 因为有病， 他觉得做不了那件事。
3. as 引导原因状语从句时表示附带说明的“双方已知的原因”， 语气比since弱， 较为正式， 位置较为灵活(常放于主句之前)。例如： As it is raining, you’d better take a taxi. 既然在下雨， 你乘出租汽车。// As you are tired, you had better rest. 既然累了， 你休息一下。// I went to bed early, as I was exhausted. 我睡得早， 因为我筋疲力尽了。
4. for引导的原因状语从句并不说明主句行为发生的直接原因， 只提供一些辅助性的补充说明， for引导的原因状语从句只能放于主句之后并且必须用逗号将其与主句隔开。例如： He could not have seen me, for I was not there. 他不可能见过我， 因为我不在那里。
［考题1］ ____ you’ve got a chance, you might as well make full use of it. (1999)
A. Now that B. After C. Although D. As soon as
［答案］ A
［解析］ 主句与从句之间存在因果关系， 且“you’ve got a chance”表示一个显而易见的原因， 因此应选用表原因的now that。
［考题2］ He found it increasingly difficult to read, ____ his eyesight was beginning to fail. (2006北京)
A. and B. for C. but D. or
［答案］ B
［解析］ “his eyesight was beginning to fail”是“he found it increasingly difficult to read”的原因， 因此本题应选用可以表示原因的连词for引导原因状语从句。
［考题3］ A man cannot smile like a child, ____ a child smiles with his eyes, while a man smiles with his lips alone. (2006湖南)
A. so B. but C. and D. for
［答案］ D
［解析］ 下划线处之后的句子补充说明“a man cannot smile like a child”的原因， 应选用for表原因。
