原因状语从句（原因状语从句讲解）
原因状语从句：1、定义：在句中用来说明主句原因的句子叫原因状语从句 。
2、常用引导词: because (因为), as (由于), since (既然百思特网), now (that) (既然)
3、时态：原因状语从句一般都是根据实际情况选用适当的时态 。通常是主过从过 ， 主现从现 。
4、because, since和as的区别：
1) because引导的原因状语从句一般放于主句的后面 ， because从句位于句首时要用逗号分开 ， 放在句末时 ， 可不用逗号分开 。because表示直接原因 ，  语气最强 ，  最适合回答why引导的疑问句 。because of 也表示原因 ， 但它后面不接从句, 只能接名词, 代词或动名词 。注意：because 和so 不可同时出现在一个句子里 。

【原因状语从句|原因状语从句讲解】

① I do it because I like it. = I like it so I do it.
② We went by bus because it was cheaper. = It was cheaper so we went by bus.
③ He can’t go to school because of his illness.
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”= now that ) ，  较为正式 ，  语气比because弱 。
① Since you are free today, you had better help me with my mathematics.
② Since you don't trust him, y百思特网ou should not employ him.
③ Now (that) you are grown up, you should not rely on your parents.
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” ， 含有对比说明的意味 ，  语气比since弱 ，  较为正式 ，  位置较为灵活(常放于主句之前) 。
①百思特网 As it is raining, you’d better take a taxi.
② As you are tired, you had better rest.
③ I went to bed early, as I was exhausted.
4) for引导的是并列句表示原因但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引导的并列句只能放于主句之后并且必须用逗号将其与主句隔开 。
① He could not have seen me, for I was not there.
② He seldom goes out now, for he is very old.