介词又叫前置词，是一种虚词。介词分为三种，一种是简单介词，如at, in, on, beside, to , for等；另一种是短语介词，即由两个以上的词组组成的短语，如in front of, because of, out of, instead of等；还有一种叫二重介词，如until after, from behind等。
（一）介词的句法功能
介词不能独立在句中做成份，介词后必须与名词、代词、或动名词构成介词短语在句中充当一个成份，表示人、物、事件等与其它人、物、事件等之间的关系。
1、作定语：The book on the table is mine.
2、作状语：We have breakfast at sev en.（表时间）；They were late for meeting because of the heavy rain.（表原因）；They started the ma chine by pressing the button.（表方法）
3、作表语：My dictionary is in the bag.
4、作宾语补足语：I found him in the office.
（二）主要介词区别
1、表示时间的at, in, on：at表示片刻的时间，如：at 8 o’clock ，常用词组有：at noon, at night, at midnight, at the end of, at that time, at the beginning of, at the age of, at Christmas, at New Year等。in表示一段的时间，如：in the morning, in the afternoon, in the evening, in October, in 1998, in summer, in the past, in the future等。on总是跟日子有关，on Monday, on Christmas morning, on the following, on May Day, on a warm morning等。
2、表示时间的since和from：since表示从过去到现在的一段时间的过程，常与现在完成时连用。from表示从时间的某一点开始，不涉及与现在的关系。一般多与现在时、过去时、将来时连用。如：I hope to do morning exercises from today./ We have not seen each other since 1995.
