原因狀語從句：
1、定義：在句中用來說明主句原因的句子叫原因狀語從句。
2、常用引導詞: because (因為), as (由於), since (既然), now (that) (既然)
3、時態：原因狀語從句一般都是根據實際情況選用適當的時態。通常是主過從過，主現從現。
4、because, since和as的區別：
1) because引導的原因狀語從句一般放於主句的後面，because從句位於句首時要用逗號分開，放在句末時，可不用逗號分開。because表示直接原因， 語氣最強， 最適合回答why引導的疑問句。because of 也表示原因，但它後面不接從句, 只能接名詞, 代詞或動名詞。注意：because 和so 不可同時出現在一個句子裡。
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引導的原因狀語從句一般放於主句之前表示已知的、 顯然的理由(通常被翻譯成“既然”= now that )， 較為正式， 語氣比because弱。
① Since you are free today, you had better help me with my mathematics.
② Since you don't trust him, you should not employ him.
③ Now (that) you are grown up, you should not rely on your parents.
3) as 引導原因狀語從句時表示附帶說明的“雙方已知的原因”，含有對比說明的意味， 語氣比since弱， 較為正式， 位置較為靈活(常放於主句之前)。
① 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引導的是並列句表示原因但並不說明主句行為發生的直接原因， 只提供一些輔助性的補充說明， for引導的並列句只能放於主句之後並且必須用逗號將其與主句隔開。
① He could not have seen me, for I was not there.
② He seldom goes out now, for he is very old.
閱讀更多 傑傑英語 的文章
