原因狀語從句：because, since,as和for的區別

原因狀語從句雖然沒有其他狀語那麼複雜，連詞總類也比不上其他狀語從語那麼多，但是也是有一些容易混淆的連詞，because，for，since，as在原因狀語從句中意思幾乎一樣，翻譯成中文也往往都翻譯成”因爲“，然而這四個詞在使用上其實也是有着一些微妙的差別的。

一. 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裏。
I do it because I like it. = I like it so I do it.
We went by bus because it was cheaper. = It was cheaper so we went by bus.
He can’t go to school because of his illness.

二. 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 )， 較爲正式， 語氣比because弱。
Since you are free today, you had better help me with my mathematics.
Since you don't trust him, you should not employ him.
Now (that) you are grown up, you should not rely on your parents.

三. 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， 較爲正式， 位置較爲靈活(常放於主句之前)。
As it is raining, you’d better take a taxi.
As you are tired, you had better rest.
I went to bed early, as I was exhausted.

四. for引導的是並列句表示原因但並不說明主句行爲發生的直接原因， 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。
He could not have seen me, for I was not there.
He seldom goes out now, for he is very old.

怎麼樣，大家都get到了這四個詞之間的細微差別嗎？文字說明看起來可能有些複雜，但多讀幾遍例句就能很快的理解了！大家在學習的過程中要多加註意哦，千萬不能放過一絲一毫的小細節！