介词的概念与分类
介词又称前置词，是一种虚词，不能单独用作句子成分，通常用于名词或代词（以及相当于名词的其他词语）之前，表示其后词语与句中其他词语之间的关系。英语介词从结构上看可以分为简单介词、双重介词和短语介词三类。
1. 简单介词
所谓简单介词就是由一个单词构成的介词。它又分为普通介词（at, by, by, for, in, over, to, up等）、合成介词（onto, into, within, without等）、分词介词（concerning, considering, including等）三类。如：
The moon was low in the sky. 月亮低低地悬在天上。
It’s a long time since breakfast. 吃早饭到现在已有很长时间了。
You can’t get into the place without a pass. 没有通行证你就进不去。
Everybody here has the influenza including myself. 包括我在内所有人都患上流感。
2. 双重介词
所谓双重介词就是指由两个简单的介词重叠使用而构成的，如from behind, from among, since before, until after, except on等。如：
A man stepped out from behind the wall. 一个人从墙后走出来。
He cannot spare any time except on Sunday. 除星期日外，他抽不出时间。
I won’t go home until after the exam. 我要等考完后再回家。
He has known her since before the war. 自战前开始他就认识她。
3. 短语介词
所谓短语介词就是指由两个或两个以上的次所组成的短语构成的。一般来说，这个短语的最后一个词是简单介词，如in case of, according to, because of, by means of, in spite of, instead of, on behalf of, with reference to等。如：
I keep an umbrella here in case of rain. 我在这里放一把雨伞以防下雨。
But things do not go according to plan. 但事情并未按计划进行。
Because of this we decided to leave early. 由于这个我们决定早些动身。
Thoughts are expressed by means of words. 思想是用词汇表达的。
In spite of his efforts he failed. 他虽然做了努力，还是失败了。