介词又称前置词，是一种虚词（名词，动词和形容词是实词），不能单独做句子成分，通常用于名词和代词之前。分为简单介词、双重介词，和介词短语。
简单介词
普通介词： at, by, for, in, over, to
合成介词： onto, into, within, without
分词介词： concerning, considering, including.
The moon was low in the sky.(地点介词)
It’s a long time since breakfast.(时间介词)
You cann’t get into the place without a pass.(方式介词)
Everybody here has the influenza including myself.(分词介词)
双重介词
由两个简单介词重叠使用而构成的。
A man stepped out from behind the wall.
I won’t go home until after the exam.
He has known her since before the war.
介词短语
由两个或两个以上的词所组成的短语，一般来说，这个短语的最后一个词是简单介词。
如：in case of(万一，以防), according to（根据，按照）, because of, by means of（通过，借助）, in spite of（尽管，虽然）, instead of（替代，而是）, on behalf of（为了，代表）, with reference to（参考，参照）.
Thoughts are expressed by means of words.
In spite of his efforts, he failed.
I keep an umbrella here in case of rain.
But things do not go according to plan.
Because of this we decided to leave early.
介词总结
早、晚要用in;
at黎明、午夜、点与分；
年、月、季节、周、阳光、灯、影、衣帽in；
将来时态in…以后，小处at(ariport)，大处in(BJ)；
有形with(stick)无形by(hardworking)，语言、单位、材料in;
特征、方面与方式，心情成语惯用in；
介词at和to表方向；
农场、值日on，关于、基础、靠、论著；
工具、和、同、随、具有、原因with；
海、陆、空、车、被by；
over, under正上下方，above、below则不然(不一定正上正下)；
Beyond超出、无、不能；against靠着，对与饭；
before、after表一点、ago、later表一段；
since以来during间，since时态多变换；
例句
Teachers live by selling knowledge.
Older parents often spend more time with their children.
Newman notes that Russia had a similar population density at the time.
Armed with a torch.
In spite of his repeated cries, no one heard him.
A day may begin well enough, but suddenly everythin seems to get out of control.
In response to renewed pleadings, Bruce stopped.
In the possession of…被谁持有
In great need. 大量需要。
Hi is running with Tom against the wind along the path.
Early in the morning, we started off for the mountain village.
回顾
虚词，不单独使用。
三大类。
重点关注介词短语。