表示词与词、词与句关系的词。
常用介词：on、at、in、above、by、with等；
合成类介词：inside、outside、onto、out of、within 等；
短语类介词：according to、ahead of、along with等。
一、含义
介词是一种用来表示词与词， 词与句之间的关系的词。在句中不能单独作句字成分。介词后面一般有名词代词或相当于名词的其他词类，短语或从句作它的宾语。介词和它的宾语构成介词词组，在句中作状语，表语，补语或介词宾语。
二、分类
1.常用简单介词：
about， across， against， among， after， at， behind， besides， beyond， by， concerning， beneath， between， despite， except， during， down， for， from， past， than， under， until 等。
2.合成介词：
inside， outside， onto， out of， within 等。
3.短语介词：
according to， ahead of， along with， as for， because of， be means of， due to， in spite of， on behalf of， owing to， with regard to 等。
三、介词与“……的”之关系
在英语学习中，一遇到“……的”，大家就会立刻想到’s 所有格或者of。实际上，在很多情况下，“……的”必须借助于介词才能准确表达。
● 美国的冬天→ the winter in America
● 停车场的入口→ the entrance to the parking lot
● 穿过森林的小路→ the path through the forest
● 鲁迅的著作→ the works by Lu Xun
● 水中的月亮→ the moon reflected in water
● 历史的见证→ the witness to history
● 对爱的渴望→ a longing for love
● 对我们不利的证据→ the evidence against us
● 阳光下的漫步→ a walk in the sunlight
● 追求名誉的女人→ a lady after fame
● 两人之间的争论→ an argument between the two persons
四、重要介词的重要用法
(1)from：此介词表示“to”相反的方向。
He came from London.
He went to London.
二词常搭配使用“from... to...”
We studied English from morning to afternoon.
He'll start from Beijing to Shanghai.
经典用法：
● 由于→ The girl is trembling from fear.
● 免除→ Tramps are always free from care. 流浪汉们总是无忧无虑。
● 分开→ The couple parted from each other at the airport.
● 由……制成→ The red wine is made from grapes. 红葡萄酒是由葡萄制成的。
由from引导的惯用法：
from far 从远处
from now / then on 从现在/那时起
from bad to worse 每况愈下
from time to time(occasionally 时而)
习语：
She comes to visit me from time to time.
(2)out of：表示与"into"相反的语意。
He will be out of town.
I stepped out of the dark room.
out of+ 名词 (= lack， to be without 用完，用光)
He went to the shop because he was out of paper.
out of date (= old-fashioned 过时的)
The book has been out of date.
out of work (= jobless 失业的)
He needs money because be is out of work.
out of the question (= impossible 不可能的)
Finishing the hard work is out of the question.
out of question (= doubtless 毫无疑问的)
That he can do it well is out of question.
out of order(= not functioning 失灵的)
Her radio is out of order， so she can't listen to it.
(3)by：表示从旁经过或在……附近。
He passed by me without saying hello.
He lives by the sea.
经典用法：
● 通过→This is the nearest road by which they came.
● 在……时候→The young man works by day and steals money by night.
● 由……所生→He has two sons by her second wife.
● 抓住→She caught her child by his coat in the flood.
● 按……计算→The farmers sell their apples by the pound.
● 就(气质，特征)而言→He is an artist by temperament(气质).
用于被动语意，导出动作执行者：
The glass was broken by the boy.
by + 具体时间：
They usually have dinner by 8 o'clock.
by + 交通工具 (bus， train， plan...)：
He travelled to Paris by air/plane.
by the end of + 时间名词：到……末为止
By the end of last year， they had learnt 100 texts.
By the end of this term， they will have learnt 100 texts.
by then：到那时
He will graduate in 1999， by then he will leave Beijing.
by way of (= via)：经由，取道
They are going to the United States by way of Hong Kong.
by the way：顺便问
By the way， do you know where Mr. Li has gone?
by far：……得多(用于修饰比较级和*高级)
The book is by far the best on the subject.
by accident：不小心，与on purpose(故意地)语意相反
The wrong information was put into the computer by accident.