这四个词都可表原因，但用法有区别。

　　　　1) 如果原因构成句子的最主要部分，一般用because。因此，because引导的从句往往放在句末。例如：

　　　　I stayed at home because it rained.

　　　　---Why aren’t you going?

　　　　---Because I don’t want to.

　　　　2) 如果原因已被人们所知，或不如句子的其它部分重要，就用as或since。Since比as稍微正式一点。As和since 引导的从句一般放在句子的开头。例如：

　　　　As he wasn’t ready, we left without him.

　　　　Since I have no money, I can’t buy any food.

　　　　3) for用来补充说明一种理由，因此，for引导的从句几乎可以放在括号里。For引导的句子不放在句子的开头。例如：

　　　　I decided to stop and have lunch----for I was feeling quite hungry.

　　　　(3)if, whether

　　　　if和whether都可作“是否”讲，在引导宾与从句是一般可互换。例如：

　　　　I wonder whether (if) you still study in that school.

　　　　I don’t know whether (if) he likes that film.

　　　　在下列情况下，只能用whether,不能用if：

　　　　1) 引导主语从句时。例如：

　　　　Whether he will come to the party is unknown.

　　　　2) 引导表语从句时。例如：

　　　　The question is whether I can pass the exam.

　　　　3) 在不定式前。例如：

　　　　I haven’t made up my mind whether to go there or not.

　　　　(4)so…that, such...that

　　　　1) so…that中的so是个副词，其后只能跟形容词或副词，而such...that中的such是个形容词，后接名词或名词短语。例如：

　　　　I’m so tired that I can’t walk any farther.

　　　　It was such a warm day that he went swimming.

　　　　2) 如果在名词之前有many, much, little, few时，用so,不用such。例如：

　　　　He has so little education that he is unable to get a job.

　　　　I have had so many falls that I am black and blue all over.

　　　　(5)either…or…, neither…nor, not only…but also…

　　　　这三个连词词组都可连接两个并列成分。当它们连接两个并列主语时，

　　　　谓语动词要随相邻的主语变化。例如：

　　　　Either you or he is wrong.

　　　　Neither he nor his children like fish.

　　　　Not only the teacher but also the students want to buy the book.

　　　　(6)although, but

　　　　这两个连词不能用在同一个句子中。例如：我们不能说“Although he is

　　　　over sixty, but he works as hard as others.”这个句子应改为：Although he is over sixty, he works as hard as others.或He is over sixty, but he works as hard as others.

　　　　(7)because, so

　　　　这两个连词同样不能用在同一个句子中。例如：我们不能说“Because John

　　　　was ill, so I took him to the doctor.” 这个句子应改为Because John was ill, I took him to the doctor.或John was ill, so I took him to the doctor.