on,at,in这三个常用介词都可以表示时间和地点,但具体用法不同,不少初学者对它们混淆不清.现在请读、记下面两个口诀,只要记住了口诀,就可避免at,on,in的种种误用.
（1）口诀之一（on,in,at表时间）.
on“在具体某一天”①
“当某时”,动名词前,arrival,death记心间②；
early,late位句先,用in一般“上”“下”“晚”③④；
at是个时间点,“工作”“时刻”与“圣诞”⑤.
注：
①on表示在具体某一天及具体某一天的上午、下午和晚上.例：
On Children's Day,all the boys and girls are wearing their newclothes．儿童节,所有的孩子们都穿上了新衣服.
②On my arrival home,I found he had gone already．当我到家时,我发现他已经走了.
③当early,late用于句首修饰介词短语时,尽管表示具体某一天的上午、下午、晚上,都要用in（如例句1）,泛指一般的上、下午,晚上也用in（如例句2）.例：
1）Early in the morning of National Day,I got up to catch the firstbus to the zoo．国庆节一清早,我便起床去赶到动物园的第一班公共汽车.
2）My father begins work at 8：00 in the morning and stops workat 4：00 in the afternoon．我父亲上午8点上班,下午4点下班.
④用于将来时态表示“过一段时间后”（如例句1）及表示“在……期间”（如例句2）和“在某个季节,某年、某月”（如例句3）都用in.请看如下例句：
1）I hear he'll be back in a month．我听说他将于一个月后回来.
2）In the course of the last lesson in French,little Franz was listen－ing to the master very attentively．在那最后一堂法语课中,小弗朗兹非常用心地听着老师讲.
3）Comrade Mao Zedong was born in December of 1893．毛泽东同志生于1893年12月.
⑤在表示某时某刻及在work,Christmas前用at.例如：
1）We get up at eight o'clock．我们8点起床.
2）My father are busily at work all day．我父亲整天忙于工作.
3）In western countries children get present from their parents atChristmas．在西方国家,孩子们在圣诞节得到父亲给的礼物.
（2）口诀之二.
at用于点,on用于天,in用于月、季、年.
午别晚上in常用,限定三时in要变.
at noon（night）,in the day,习惯用语记心间.
说明：
当 morning,afternoon,evening有前位定语或后置定语限定时,就不用in而用on.例：
on a cold（winter）evening在一个寒冷（冬天）的晚上
on Wednesday afternoon在星期三下午
on the morning of March 8th在3月8日上午
下面再说说这三个介词在表示地点时用法的区别,口诀如下：
on,in,at表地点,on“在之上”接触面,
“靠近、接壤、左右边”①； in“在里面”和“中间”②；
at表示小地点,“入口、车站、影剧院”③；
home出现定语、冠,须用in把at换④；
“夜间、车辆”若有限,及“在途中”on在前⑤.
说明：
①在表示地点时,指“在……之上（与表面接触）”.例：
There is an English－English dictionary and two grammar books onthe big desk．那张大写字台上有一本英语词典和两本语法书.
“靠近、接壤”及“在左右边”都用on.例：
Korea lies on the northeast of China．朝鲜位于中国东北方.
Tom was sitting on my left when we saw the film yesterday．我们昨天看电影时,汤姆坐在我左边.
②表示“在……里面”（即物体内部）.例：
Jim turned the key in the lock and opened the door．吉姆把钥匙插在锁孔里,转动一下,打开了门.
“在……中间”（即middle前）都用in.例：
Soon they were in the middle of the river．很快他们就游到了河中间.
③at表示一个较小的地点,如在“入口”（entrance）“车站”（busstop,railway station）和“影剧院”（cinema,theater）等名词前.例：
As she walked along,she noticed an old pine tree ahead at the en－trance to a valley．她往前走时,注意到前边峡谷口有棵老松树.
④home前一般用at,但若有物主代词和冠词等定语修饰时,须用in.例：
In Princeton he lived quietly,working at the institute and enjoyinghimself by playing his violin in his simple home．他在普林斯顿过着安静的生活,在研究所工作,在他那简朴的家里拉琴消遣.
⑤“夜间”（night）、“车辆”（bus,bike,train ect．）等名词若有限定词及“在……途中”,介词则用on.例：
1）He was in on that night．他那天夜里在家.
2）My son often goes to school on his bike．我儿子常骑自行车去上学.
3）On the way home my father told me of an incident that tookplace on his first day at school in Mr Crossett's class．在回家途中,父亲给我讲了他第一天上克罗塞特先生课时发生的一件事.1、Lucy can write a letter___Japanese though she has learned it only a few months.
A.from B.to C.in D.with
2、They will have an English test___two days.
A.for B.at C.in D.after
3、Wu Dong was born___the evening of April 2,1975.
A.at B.in C.on D.to
