conjunction
连词是一种虚词， 它不能独立担任句子成分而只起连接词与词，短语与短语以及句与句的作用。连词主要可分为两类：并列连词（coordinating conjunctions）和从属连词（subordinating conjunctions）。
并列连词用来连接平行的词、词组和分句。如：and, but, or, nor, so, therefore, yet, however, for, hence, as well as, both…and, not only…but also, either…or, neither…nor, (and) then等。
上述并列连词又可分为: 一般连词（simple conjunction） , 关联连词（correlative conjunction）
coordinating conjunct（并列连词）
, or, not only… but also, as well as等表示并列
She plays (both) the piano and the guitar. = If you make up your mind, you'll get the chance.
Make up your mind, and you'll get the chance.
There is no air or water in the moon. =There is no air and no water on the moon.
She plays not only the piano but (also) the guitar.
Neither you nor he is to blame.
注意：
(1) not only… but also 关联两个分句时，一个分句因有否定词not 而必须倒装。如:Not only does he like reading stories, but also he can even write some.
(2)neither…nor 意思为"既不……也不……"谓语动词采用就近原则，与nor后的词保持一致。请选择：
---I don‘t like chicken ___ fish.
---I don’t like chicken, ___ I like fish very much.
A. and； and B. and； but
C. or； but D. or；and
改错：
We will die without air and water.
We can't live without air or water.
2.表示选择
or意思为“否则”。
eg. I must work hard, or I'll fail in the exam.
either…or 意思为“或者……或者……”。注意谓语动词采用就近原则。
eg. Either you or I am right.
3.表示转折或对比
but表示转折，while表示对比。
选择比较下面两句：
(1) Some people love cats, ______ others hate them.
(2) --- Would you like to come to dinner tonight
--- I‘d like to, ______ I’m too busy.
A. and B. so C. while D. but
not…but… 意思为“不是……而是……“ not 和but 后面的用词要遵循一致原则。
eg. They were not the bones of an animal, but (the bones) of a human being.
4.表原因
for是并列连词，不能置于含两个并列分句的句子的句首，只能将其放在两个分句中间。
so, therefore
eg. He hurt his leg, so he couldn‘t play in the game.
注意：两个并列连词不能连用，但therefore, then, yet例外。
eg. You can watch TV, and or you can go to bed.(错)
He hurt his leg, and therefore he couldn‘t play in the game.(对)
改错：For he is ill, he is absent today.
Although he was weak, but he tried his best to do the work.
subordinating conjunction（从属连词）
英语从属连词( subordinating conjunction)用来连接各种从句。由从属连词引导的句子通常叫从句，而把含有从句的句子叫作复合句。从属连词大致可分为如下三大类：
引导状语从句的从属连词
引导名词性从句的从属连词
引导定语从句的从属连词
1. 引导时间状语从句的从属连词
(1) 表示“当…时候”或“每当”的时间连词。主要的有 when, while, as, whenever：
He jumped up when the phone rang. 电话铃响时他吓了一跳。
We listened while the teacher read. 老师朗读时我们听着。
The phone rang just as I was leaving. 我正要离开，电话铃就响了起来。
(2) 表示“在…之前(或之后)”的时间连词。主要的有before, after：
Turn the lights off before you leave. 离开前请关灯。
He started the job soon after he left the university. 他大学毕业后就开始做这份工作。
(3) 表示“自从”或“直到”的时间连词。主要的有since, until, till：
He has lived here since he got married. 他结婚后就一直住在这儿。
Most men worked until [till] they’re 65. 大多数男人工作到65岁。
温馨提示:
用until / till引导时间状语从句时，主句的谓语动词要十分小心地去加以选择的。如谓语动词是持续性的, 要用肯定形式，如：I studied hard until 12 o‘clock last night.如果谓语动词是瞬间性的，则要用否定形式，如:He didn't go to bed until his mother came back.
(4) 表示“一…就”的时间连词。主要的有as soon as, the moment, the minute, the second, the instant, immediately, directly, instantly, once, no sooner…than, hardly…when等：
Tell him the news as soon as you see him. 你一见到他就把这消息告诉他。
I recognized her the moment (that) I saw her. 我一看到她就认出她来了。
I want to see him the minute (that) he arrives. 他一到来我就要见他。
I went home directly I had finished work. 我一干完活就回家了。
Once he arrives, we can start. 他一来我们就可以开始。
(5) 表示“上次”、“下次”、“每次”等的时间连词。主要的有every time(每次)，each time(每次)，(the) next time(下次)，any time(随时)，(the) last time(上次)，the first time(第一次)：
Last time I saw him, he looked ill. 上次我见到他的时候，他好像有病。
Do look me up next time you’re in London. 你下次到伦敦来，一定来我。
Every time I call on him, he is out. 我每次去访问他，他都不在。
You can call me any time you want to. 你随时都可以给我打电话。
2. 引导条件状语从句的从属连词:主要有if, unless, as (so) long as, in case 等：
event是什么词性If anyone calls tell them I’m not at home. 要是有人打电话来，就说我不在家。
You will fail unless you work hard. 你若不努力就会失败。
As [So] long as you need me, I’ll stay. 只要你需要我，我就留下。
In case I forget, please remind me about it. 万一我忘记，请提醒我一下。
【注】在条件状语从句中，通常要用一般现在时表示将来意义，而不能直接使用将来时态。不过，有时表示条件的 if之后可能用 will，但那不是将来时态, 而是表示意愿或委婉的请求(will为情态动词)：
If you will wait a moment, I’ll fetch the money. 请等一下，我就去拿钱。
3. 引导目的状语从句的从属连词:主要的有 in order that, so that, in case, for fear等：
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