1.定义：连接两个句子的单词或短语；
2. 用法：
(1)并列连词：and, but, or;
①and ”和，而且”，前后状况一样或相似；用于肯定句；
e.g. Tom likes basketball and I also like basketball.
Tom is outgoing and hardworking.
=Tom is outgoing and he is also hardworking.
②but “但是”,表示转折，前后状况不一样或是相反；
e.g. Tom likes basketball, but I like baseball.
Tom is hardworking, but he is careless.
③or “或者”，表示选择关系；通常用于否定句和一般疑问句；
e.g. Tom doesn’t like apples or oranges.
Would you like to drink, juice or water?
④and和or 的特殊用法：
and表示递进，“这样做的话，就会怎么样”，结构为“祈使句，and+句子（用将来时）”；
e.g.Work hard, and you will get good grades.
Stay here, and you will know the truth.
or表示否则，“这样做的话，否则会或不会怎么样”，结构为“祈使句，or+句子（用将来时）。
e.g. Work hard, or you won’t get good grades.
Take the umbrella, or you will get wet.
(2)时间连词：when, while, as, since, before,after, as long as等。
①when, while和as都意为“当……的时候”；引导时间状语从句；
②when引导的从句谓语动词可以为延续性动词和非延续性动词；
③while引导的从句谓语动词必须为延续性动词，通常用于进行时；
④as引导的从句谓语动词通常为延续性动词，通常用于进行时；还可以表示“一边……一边……”。
e.g. I am doing my homework when Tom comes back.
I was doing my homework while/when /as my mother was doing breakfast yesterday morning.
Tom is reading a book as he is listening to music now.
⑤since意为“自从”，引导的时间状语从句用过去时，主句用现在完成时；
e.g. Tom has lived in Kunming since he came here.
⑥before 意为“在……之前”；反义词after”在……之后”，均引导时间状语从句；
e.g. Tom had lived in Kunming before he began to learn English.
She has studied French after she made the decision.
⑦as soon as意为“一……就……”, 主句通常用将来时，从句用一般现在时。
e.g. I will call you as soon as I arrive in Beijing.
(3)其它常见连词：
①if 和unless(= if not),引导条件状语从句，时态遵循“主将从现”；
e.g. You will get good grades if you work hard.
Everything will get worse unless we take action at once.
②either…or…“要么……要么……”,表示两者中选其一；谓语动词单复数取决于就近原则；
neither…nor…”既不……也不……”，表示两者都不选；谓语动词单复数取决于就近原则；
not only…but also…”不但……而且……”，谓语动词单复数取决于就近原则；
both…and…“两者都”，表示两者都选，作主语为复数。
e.g.Either you or Tom plays the role.
Neither Tom nor I am not students.
Not only Tom but also other students in my class are hardworking.
Both Tom and you are good students.
③though, although, even if, even though意为“虽然”，不与but连用，但可以与yet(但是)连用，引导让步状语从句；
e.g. Though Tom is eight years old, he can speak English well.
④because意为”因为”，引导原因状语从句；so意为“因此”，引导结果状语从句；注意because不能与so连用；
e.g.Tom went to school late because he got up late.
Tom got a cold so he should go to see the doctor.
⑤till, not…until意为“直到……才…”,引导时间状语从句；
e.g. Tom didn’t come back home until I finished my homework.
⑥ as long as”只要”；
e.g. As long as you stay here, you will know the thing.
⑦while和but都表转折时的区分：but只表示前后状况相反或不一样，而while表转折时有鲜明对比在里面；
e.g. Some people are rich while/but others are poor.
⑧since意“因为”，相当于because;意为“既然”，相当于now that。
e.g. Since she said like this, I think we should believe her.
Don’t criticize him like that since he is only a child.
