1、介詞是一種虛詞。不能單獨作句子成分，它只有跟它後面的賓語一起構成介詞短語，才能在句子中起作用。
有：in, on, under, with, behind, about, near, before, after, for, to, up, down, from, in front of, out of, fromto, at the back of
2、表示時間的'介詞有：at, on, in。
(1)at表示在某一個具體的時間點上，或用在固定片語中。如：at ten oclock, at 9:30 a.m., at night, at the weekend
(2)on表示在某日或某日的時間段。如：on Friday, on the first of October, on Monday morning
(3)in表示在某一段時間(月份、季節)裡。如：in the afternoon, in September, in summer, in 2005
3、in一詞還有其他的固定搭配，如：in blue(穿著藍色的衣服)，in English(用英語表達)，take part in(參加)。
練一練：
1、選用括號內恰當的介詞填空。
1) Whats this _______ ( at, on, in ) English?
2) Christmas is _______ ( at, on, in ) the 25th of December.
3) The man_______ ( with, on, in ) black is Su Hais father.
4) He doesnt do well _______ ( at, on, in ) PE.
5) Look at those birds _______ ( on, in ) the tree.
6) We are going to meet _______ ( at, on, in ) the bus stop _______ ( at, on, in ) half past ten.
