连词：是连接单词、短语或句子的一种虚词，在句中不单独作任何成份，一般不重读。
一. 连词的分类
1. 按连词的构成可将其分为以下几类：
1 简单连词，如and， if， or， because， but， so
2 关联连词，如both … and …， not … but …， not only …
but also …
3 分词连词，如supposing， providing， provided， given
4 短语连词，如as if / through， even if / through， as / so long as
2. 按连词的性质可将其分为以下几类：
1 并列连词，如and， but， or， nor， so， for， as well as， while，
when， not only … but also …， both … and …，
either … or …， neither … nor， therefore， yet， nevertheless， however等。它们用来连接平行的词、词组或分句。
2 从属连词，如after， when， before， as， while， since， until， till，
although， though， if， even if， unless，
lest， because， than， that， whether， so
that， as soon as， as long as， in order that， as though， suppose (that)， provided (that)， in case (that)， now (that)， on condition (that)， seeing that， so … that， such … that， as … as， so …as等。它们用来引导从句。
温馨提示：除连词外，连接代词、连接副词、关系代词、关系副词也可以来引导一个从句。
二. 并列连词
1. 表示并列关系的连词：主要表示“和，补充，增加”之意，包括等。如and， both … and， neither … nor， not
only … but (also)， as well as等。
如Neither
Mary nor Tom came to our party yesterday.
She plays
both the piano and the guitar.
He has
been to all the countries in the Middle East as well as Europe.
He is not
only our teacher but also our friend.
温馨提示：在连接三个或三个以上的名词或相当于名词的短语时，一般应在最后两者之间加and。如He got up，washed
his face， had breakfast and rushed to school.
2. 表示转折关系的连词：主要有but， only， while， when， whereas等。
如A great
deal has been accomplished， but more remains to be
done.
He is
riding， while his father is walking in the dust.
Why did
you borrow the book when you had one？
He is ill， whereas I am only a little tired.
only表示意义转折，只见于非正式文体。如You may go， only come back early.(only =
but)
温馨提示：○1 有些词或短语在句中也表示转折意义，如still， yet， however， all the same， after all等。通常作连接性状语。
如The
problem was a little hard， yet I was able to work it
out
I
explained twice， still he couldn't understand.
2 whild在表示转折关系时，往往连接内容和结构对称的句子。
如I love
strong tea while my father loves coffee.
3 both … and … not结构和neither
… nor …的区别在于前者表示部分否定，而后者表示全部否定。如Both you and I are not correct. Neither you nor I am right.
3. 表示选择关系的连词：主要有or， either … or， or else， whether …
or， otherwise等。如Which do you
like better， tea or coffee？
Either you
leave this house or I'll call the police.
Do it at
once， or else！ = Do it at once， otherwise you will suffer in some way.
Take a
taxi， otherwise you won't get there in time.
4. 表示因果推理关系的连词：主要有so， for， then， therefore等。
如The air
here is polluted， so the crops are dying.
It must be
late， for I have been here a long time.
温馨提示：for，because，as，since在用法和意义上的区别：
1 for引导的句子是并列分句，只能后置，并用逗号与前面的句子隔开。主要用来表示推测性原因，或附带解释、说明前一分句的情况。
如The
electric current must have been turned off， for the
light went out.
2 because用来引导表示直接原因的从句，语气很强，明确地说明因果关系。其引导的从句可置于句首、句末或句中。如He is absent today because he is ill.
for引导分句表示因果关系时，它可以和because互换使用。
如上句也可以说成：He
is asent today， for he is ill.
但表示推测性原因，就不能互换使用。如He must be ill， for he is absent today.
3 since，as都是不讲自明的原因，是已知的原因，as的语气比since要弱。
如Since
(As) you don't feel well， you had better stay at home.
三. 从属连词：用来连接分句，表示各分句之间的关系。其可分为引导名词性从句、状语从句的连词和引导副词性从句的连词。用法详见“主从复合句部分”。
