4) 當(dāng)兩個短動作同時發(fā)生時，或表示“一邊…一邊…”時，最常用as。例如：

　　Just as he caught the fly， he gave a loud cry.

　　She looked behind from time to time as she went

　　5) 當(dāng)從句的動作先于主句的動作時，通常用when。例如：

　　When he finished his work， he took a short rest.

　　6) 當(dāng)從句是瞬間動作，主句是延續(xù)性動作時，通常用when。例如：

　　When John arrived I was cooking lunch.

　　(2)as， because， since ， for

　　這四個詞都可表原因，但用法有區(qū)別。

　　1) 如果原因構(gòu)成句子的最主要部分，一般用because。因此，because引導(dǎo)的從句往往放在句末。例如：

　　I stayed at home because it rained.

　　---Why aren’t you going？

　　---Because I don’t want to.

　　2) 如果原因已被人們所知，或不如句子的其它部分重要，就用as或since。Since比as稍微正式一點。As和since 引導(dǎo)的從句一般放在句子的開頭。例如：

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

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

　　3) for用來補充說明一種理由，因此，for引導(dǎo)的從句幾乎可以放在括號里。For引導(dǎo)的句子不放在句子的開頭。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”講，在引導(dǎo)賓與從句是一般可互換。例如：

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

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

　　在下列情況下，只能用whether，不能用if：

　　1) 引導(dǎo)主語從句時。例如：

　　Whether he will come to the party is unknown.

　　2) 引導(dǎo)表語從句時。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是個副詞，其后只能跟形容詞或副詞，而such...that中的such是個形容詞，后接名詞或名詞短語。例如：

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

　　It was such a warm day that he went swimming.

　　2) 如果在名詞之前有many， much， little， few時，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…

　　這三個連詞詞組都可連接兩個并列成分。當(dāng)它們連接兩個并列主語時，

　　謂語動詞要隨相鄰的主語變化。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

　　這兩個連詞不能用在同一個句子中。例如：我們不能說“Although he is

　　over sixty， but he works as hard as others.”這個句子應(yīng)改為：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

　　這兩個連詞同樣不能用在同一個句子中。例如：我們不能說“Because John

　　was ill， so I took him to the doctor.” 這個句子應(yīng)改為Because John was ill， I took him to the doctor.或John was ill， so I took him to the doctor.

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