　　一、概说
连词，是一种虚词，不能担任一个句子成分，但可以起连接的作用，即连接词与词或句与句。例如：
We may be leaving today or tomorrow. 我们可能今天或明天走。（连接词与词）
Now I must go or I shall be late for the party. 我现在得走，否则晚会我就要迟到了。（连接句与句）
主要是包括并列连词和从属连词。在句子中，连词起连接词与词、短语与短语及句子和句子的作用。它本身在句子里不单独做句子成分。
　　并列连词：是用来连接语法地位相同的结构、相同的单词、短语及句子。
　　并列连词有：and, but, or, nor, so, for yet, however, as well as, both...and, not only...but also, either...or, neither...nor, still,
　　从属连词：用来引导状语从句。
　　从属连词有：after, when, before, as, while, since, until, till, if, unless, lest, because, than, that, whether, so that, in order that, as if, as though, although, suppose that, provided, that, as...as, now that, such...that, in case that, on condition that
二、连词的种类
连词有两类：
1）从属连词：引导从句。随着从句作用不同，它们又可分为几类：
从句种类
主要从属连词
时间从句
when while as before after until till whenever
条件从句
if unless supposing provided(ing) suppose
目的从句
in order that so that that so lest
结果从句
so…that such…that so that so
原因从句
because as since
让步从句
although though even though(if) while
方式从句
as like the way as if as though
地点从句
where wherever
比较从句
than as
此外还有 that, whether 等从属连词可引导名词从句，在句中担任主语、宾语等。 2）并列连词：连接两个互不依从的次、短语或分句：
Slow but sure. 要慢而稳。（连接单词）
She’ll be back either this week or next week. 她将在这周或下周回来。（连接短语）
I went and she went also. 我去了，她也去了。（连接分句）
并列连词有下面几类：
表示意思转折的连词 but yet however nevertheless 表示因果关系的连词 for so therefore hence 其他并列连词 and or either…or neither…nor not only…but also both…and as well as
三、从属连词
1. 引导时间状语从句的连词
这类连词主要有：
when:
Don’t get excited when you talk. 讲话时别激动。
When he got up he felt dizzy. 他站起身时感到头晕。
while:
We must strike while the iron is hot. 我们要趁热打铁。
While she ate she grew more restless. 她一边吃一边变得更加忐忑不安。
as:
As he spoke two men came up. 在他讲话时两个人走了进来。
He smiled as he passed. 他经过时笑了笑。
before:
Look before you leap. 三思而后行。
It will be five years before we meet again. 要五年后咱们才能再相见。
after:
I arrived after he had left. 在他走后我到了。
I’ll tell them after you have left. 你走后我在告诉他们。
until, till :
I’ll take no steps until you arrive. 你来之前我不会采取什么行动。
I propose waiting till the police get here. 我建议等警察来了再说。
since:
How long is it since you came to London 你到伦敦有多久了？
It was years since I had seen her. 我有好多年没见到她了。
whenever:
I go and visit him whenever I’m in town. 我每次进城都去看他。
Whenever possible, they play outside. 一有机会他们就到外面玩耍。
2. 引导条件状语从句的连词
1）条件状语从句主要由if, unless, supposing(suppose)引导：
if:
I must leave if that’s the case. 如果情况如此我就得走了。
He will come if asked. 如果邀请的话他会来的。
unless:
I won’t write unless he writes first. 我不写信，除非他先给我写。
I’ll go there tomorrow unless it rains. 除非下雨，否则我明天去那里。
supposing:
Supposing that he asks you, will you go 假定他请你去，你会去吗？
Supposing his plan goes wrong, what will you do then
如果他的计划出了问题，我们怎么办？
suppose:
Suppose she finds out, what shall we do then 假如她发现了，我们怎么办？
Suppose you lost your job tomorrow, what would you do 假定你明天失业，你怎么办？
provided:
I will agree to go provided(providing) (that) my expenses are paid.
如果有人给我掏路费我将同意去。
She agreed to go and work there provided that her family could go with her.
providing:
I’ll dry the dishes, providing that you do the washing-up. 如果你刷盘子，我就将它们擦干。
2）条件状语从句还有其他形式：
You will always have a home as long as I have anything.
只要我有家产，你就永远会有一个家。
They’ll stand by you even if you don’t succeed. 即使你不成功，他们也会支持你。
此外，as long as, so long as, in case, even if 都起连词作用。
3. 引导目的状语从句的连词
引导目的的状语从句的连词主要有下面这些：
16.答案A
17.答案是D.分析句子结构会发现，空缺处需要的是表示时间状语从句的引导词，选项中只有D符合。
18.答案是B.本题查考对句子含义的理解及对强调句型的是“it is/was…that(或who)…”选项中只有B为正确答案。
19.答案A.从句意看只有A适合，now that 意为既然、由于，常放在句子开头。
20.答案A。只有why能引导原因状语从句。
21.答案是D, since的主语通常是现在完成时，而as和until又与句意不符，只能选D.