原因狀語從句詳解

　　1. because引導的原因狀語從句一般放于主句之后， because表示直接原因， 語氣最強， 最適合回答 why引導的疑問句。例如： I do it because I like it. 我做這件事是因為我喜歡。

　　注意： “not ... because”結(jié)構(gòu)中的not否定的是because引導的整個從句， 例如： The country is not strong because it is large. 國強不在大。

　　2. since引導的原因狀語從句一般放于主句之前表示已知的、 顯然的理由(通常被翻譯成“既然”)， 較為正式， 語氣比because弱。例如： Since you are free today, you had better help me with my mathematics. 既然今天你休息， 你最好幫我補習數(shù)學。

　　注意： seeing (that)， now (that)， considering (that)， in that這幾個詞匯與since引導的原因狀語從句意思相近， 都表示“既然”。例如： Seeing (that) he refused to help us, there's no reason that we should now help him. 他既然曾經(jīng)拒絕幫助我們， 我們現(xiàn)在沒有理由要來幫助他。// Now (that) you are grown up, you should not rely on your parents. 既然你長大了， 就不應(yīng)該依靠你的父母了。// Considering (that) everybody is here, let's begin our discussion. 既然大家都到了， 我們就開始討論吧。// In that he is ill, he feels unable to do it. 因為有病， 他覺得做不了那件事。

　　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”， 語氣比since弱， 較為正式， 位置較為靈活(常放于主句之前)。例如： As it is raining, you'd better take a taxi. 既然在下雨， 你最好乘出租汽車。// As you are tired, you had better rest. 既然累了， 你最好休息一下。// I went to bed early, as I was exhausted. 我睡得早， 因為我筋疲力盡了。

　　4. for引導的原因狀語從句并不說明主句行為發(fā)生的直接原因， 只提供一些輔助性的補充說明， for引導的原因狀語從句只能放于主句之后并且必須用逗號將其與主句隔開。例如： He could not have seen me, for I was not there. 他不可能見過我， 因為我不在那里。

　　[考題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] 主句與從句之間存在因果關(guān)系， 且“you've got a chance”表示一個顯而易見的原因， 因此應(yīng)選用表原因的now that。

　　[考題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， 因此本題應(yīng)選用可以表示原因的連詞for引導原因狀語從句。

　　[考題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] 下劃線處之后的句子補充說明“a man cannot smile like a child”的原因， 應(yīng)選用for表原因。