介词是英语中最活跃的词类之一，大部分习惯用语都是由介词和其它词类构成，且在文章、练习和考试中大量出现。掌握好介词是学好英语的一个重要关键。
介词大致可分为三类：简单介词、合成介词和成语介词。简单介词有about；across；after；against；among；around；at；before；behind；below；beside；between；by；down；during；for；from；in；like；of；off；on；over；near；past；round；since；through；till；to；towards；under；until；with等；合成介词有into；out of；outside；without等；成语介词有in front of；instead of等。
介词是一种虚词，用来说明名词和其它词之间关系。介词不能在句中独立充当一个成分，而需和名词等构成介词短语，介词短语在句中可以作定语、状语、表语，宾语补足语等。
（一）表示时间的介词
（1）at，on，in
①表示时间“点”用at。例如：at seven，at noon等。有时表示不确定的时间和短期假日的名词之前也可用at。如at that time，at night等。例如：
—When did Mr．GREen come here？
—At nine o'clock yesterday morning．
Wu Yin gets up at six o'clock every day．
②表示时间“段”用on或in，一般说来，在某一天用on。如on Monday，on June 1st；指在一段时间内（年、月、日等）用in。如in the morning，in a week，in summer，in 1999，in October等。例如：
When were you born？I was born on September 6，1983．
He woke up several times in the night．
I was born in August，1985．
Mrs Brown came to China in 1996．
③表示某天的morning，afternoon，evening时用on。如果在morning，afternoon，evening之前有形容词修饰也用on。例如：
I remember Susan left on a very cold morning of January．
—What will you do on Christmas Eve？
—We'll have a big party．
（2）during，for，since，from
①during表示某一特定的一段时间，通常与一般过去时连用。例如：
She woke many times during the night．
②for表示好长一段时间，指时间上的延续。例如：
My uncle has worked in this hospital for ten years．
③since后接时间的起点，其终点往往是现在，常与完成时连用。例如：
I haven't seen you since last Saturday．
Tom has worked here since two years ago．
He has been late for school three times since the morning of last Friday．
（3）after，in
这两个介词都可表示时间，意为“在……以后”。它们的区别是： after以过去为起点，表示过去的一段时间之后，通常用于过去的时态；in以现在为起点，表示将来一段时间之后，常用于将来的时态。例如：
After gratuation，he went to Japan．
The teacher will be back in an hour．
（二）表示地点、位置的介词
（1）in，on，to，at
这四个介词都表示地理位置的关系。in表示在某一范围内；on表示两个不同的个体相邻或接壤；to则表示两个个体之间有一段距离或隔海相望；at表示在街道或城镇。例如：
Jim GREen lives at 152 Jianguo Street，not far from the center of the city．
This kind of computer is made in Beijing．
Leave the glasses on the table．
Hainan lies to the south of Guangdong．
（2）above，over，on
这三个词都表示一个物体高于另一个物体，可译作“在……上面”。above表示位置高于某物，在其上方，但不是正上方。它的反义词是below；over表示在正上方，指垂直方向，它的反义词是under； on指两个物体表面接触，一个在另一个的上面。例如：
Let me show you the place on the map．
The water came above our knees．
A lamp was hanging over the table．
It's quite cold today．The temperature will fall below zero at night．
（3）between，among
between一般指两人或两物之间。among指三个或三个以上的人或物之间，其宾语是名词或代词。例如：
Don't tell anybody about it．Keep it between you and me．
Pass out the books among the students．
（4）across，through
across指从一边到另一边，“横过，越过”物体的表面；through意为“穿越，通过”空间。例如：
China has built a GREat Green Wall across the northern part of the country．
Light comes in through the window．
（三）表示方式的介词by，with，in
by表示“以……方式（方法或手段）”和“乘坐某种交通工具”； with指借助于某种具体的手段或工具”；in表示“以……方式，用……（语言、文字等）媒介”。例如：
What do you mean by“take-away”，please？
What's this in Chinese？
The woman in a red dress is my aunt．
“All of us are good at English now，”the teacher said with a smile．
With the help of my teacher，I did well in maths．
（四）表示“除去”的介词except，besides，but
（1）except通常跟名词或代词、或跟介词短语或从句，作“除了……以外不再有”或“不把……包括在内”讲，即在整体中除去一部分，有时用于all，none等词的后面。例如：
Everybody is here except Li Lei．Where is he？
All of us passed the exam except Tom．（汤姆没有通过考试。）
（2）but多和nobody，nothing，who，no one，everyone，all等连用，与 except意思相近，意为“除了……以外”，后面跟名词或代词。例如：
No one but me went to see him yesterday．
（3）besides意为“除了……以外还有”。例如：
Wei Hua came late to the meeting besides Lucy．（魏华和露茜都迟到了。）
（五）介词的习惯用法
（1）to。例如：
He give me two answers to the questions，but neither of them is right．
Thanks to your help，we finished the work on time．
（2）of。例如：
These knives are made of metal and wood．
The room was full of smoke after the big fire．
（3）on。例如：
You must come to work on time．
Mr．Bo Xilai gave the American friends a talk on our city．
（4）in。例如：
The little boy is always interested in science．
In the end，the lost child was found by a policeman in a small village．
Since you are in trouble，why not ask for help？
（5）from。例如：
This programme was sent to the USA from China by satellite．
No one can stop her from leaving for Shanghai．
Our English teacher comes from England．
（6）for。例如：
China is famous for the GREat Wall．
Here are some presents for you with our best wishes．
Ann is often late for school．
（7）with。例如：
—What have you done with the letter？
—I have posted it．
She is very patient with the children．
