介词(preposition)：也叫前置词。在英语中，它不能单独充当句子成分，需要和名词或代词（或相当于名词的其他词类、短语及从句）构成介词短语，才能在句中充当成分。
一、介词的分类
1. 介词不能单独作句子的成分，而是用来表示名词或代词等和句中其他词之间的关系，通常放在名词或代词之前，构成介词短语。根据结构，介词主要分成四种：
1）简单介词：about, across, after, against, among, around, at, before, behind, below, beside, but, by, down, during, for, from, in, of, on, over, near, round, since, to, under, up, with等等。
2）合成介词：inside, into, outside, throughout, upon, without, within
3）短语介词：according to, along with, apart from, because of, in front of, in spite of, instead of, owing to, up to, with reguard to
4）分词介词：considering, reguarding, including, concerning
2. 从意思上考虑，介词可分为下面三类：
1）引导时间短语的介词：
at, on, in, during, before, after, over, from, following, by, till, until, towards, for, throughout, upon, since, pending, all through, prior to
2) 引导地点状语的介词：
in, at, across, to, down, under, near, between, over, from, into, through, onto, off, outside, out of, inside, within, beside, behind, below, among, beyond, against, around, before, up, ahead of, via, in front of, under, along, past, opposite, above, next to
3) 引导其他短语的介词：
by, with, about, except, like, of, according to, instead of, without, in, due to, because of, along with, as for, in spite of, on account of owing to, on behalf of, against, apart from, for, failing, plus, including, in regard to, with regard to
二、介词短语在句子中的作用
1. 用作状语
介词短语表示时间、地点、方式和原因等。如：
Class begins at half past seven. (时间) 7时半开始上课。
His parents work on a farm. (地点) 他父母在农场工作。
She came here by train. (方式) 她乘火车来这里。
You don’t have to worry about that. (原因) 你不必为那件事担心。
2. 用作定语
作定语用的介词短语应放在所修饰的名词之后。如：
The book on the desk is very interesting. 书桌上的那本书很有趣。
The young man from Guilin is my uncle. 从桂林来的那位年轻人是我叔叔。
We visited the museum in the centre of the city. 我们参观了市中心的博物馆。
3. 用作表语
在系动词后的介词短语用作表语。如：
My father was in the army two years ago. 两年前我父亲在部队。
This book will be of great use to you. 这本书对你会有很大的用处。
