介词 (preposition) 又称前置词，是英语中最活跃的词类之一，它表示的是它后面的名词或代词(或相当于名词的其他词类、短语或从句)与其他句子成分的关系。 介词可分为下列三类： 1. 简单介词，如： about, above, across, after, against, among, at, before, behind, below, beneath, beside, besides, beyond, but, by, down, during, except, for, from, in, like, near, of, off, on, over, past, round, since, through, to towards, under, until, till, up, with 2. 合成介词，如： inside, into, onto, outside, throughout, upon, within, without 3. 短语介词，如： according to, along with, apart from, as for, as to, because of, by means of, in front of, in spite of, instead, in accordance with, on account of, on behalf of , owing to, due to, together with, up to, with regard to, next to, prior to 介词不能单独充当句子成分，它必须与名词或相当于名词作用的成分构成介词短语才能充当 句子的成分。 二、介词短语的句法功能 1. 作定语，例如： I have lost the key to the door of my room. 我把房间的钥匙丢了。 还有很多。。。。。
在定语从句“介词+who/which”的结构中,不能用that 代替who/which.She is a good student from who we should learn.in , on,at 在……时 in表示较长时间,如世纪、朝代、时代、年、季节、月及一般（非特指）的早、中、晚等.
如 in the 20th century, in the 1950s, in 1989, in summer, in January, in the morning, in one’s life , in one’s thirties等. on表示具体某一天及其早、中、晚. 如on May 1st, on Monday, on New Year’s Day, on a cold night in January, on a fine morning, on Sunday afternoon等.
at表示某一时刻或较短暂的时间,或泛指圣诞节,复活节等. 如at 3:20, at this time of year, at the beginning of, at the end of …, at the age of …, at Christmas, at night, at noon, at this moment等. 注意：在last, next, this, that, some, every 等词之前一律不用介词.
3、重叠介词：from among 从...当中；from behind 从...后面；until after 直至...之后；at about 在大约；after about 在大约...之后 等。
4、短语介词：一个或两个简单介词和一个或几个其他词类构成一个短语，作用相当于一个介词，这就叫做短语介词。这类介词的末尾总是一个简单介词；如 according to，because of，by means of，in addition to，in front of， in spite of，into等。
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