1. because引導(dǎo)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(qiáng)，最適合回答 why引導(dǎo)的疑問句。例如：I do it because I like it. 我做這件事是因?yàn)槲蚁矚g。
注意：“not ... because”結(jié)構(gòu)中的not否定的是because引導(dǎo)的整個(gè)從句，例如：The country is not strong because it is large. 國強(qiáng)不在大。
2. since引導(dǎo)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(bǔ)習(xí)數(shù)學(xué)。
注意：seeing (that)，now (that)，considering (that)，in that這幾個(gè)詞匯與since引導(dǎo)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?yàn)橛胁。X得做不了那件事。
3. as 引導(dǎo)原因狀語從句時(shí)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?yàn)槲医钇ＡΡM了。
4. for引導(dǎo)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(bǔ)充說明，for引導(dǎo)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?yàn)槲也辉谀抢铩?br />
[考題1] ____ you’ve got a chance, you might as well make full use of it.
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(gè)顯而易見的原因，因此應(yīng)選用表原因的now that。
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