1.at

　　1）用于表示在时间的某一点或某一时刻.例如:

　　Whatwereyoudoingatthistimeyesterday?

　　Studentsoftenstartclassesateighto'clockinthemorning.

　　2）用于表示在三餐或中午、晚上.例如:

　　Whatdidyourmothersayatbreakfast／lunch／suppertime?

　　Weheardagirlsinginginthenextroomatnoon／night.

　　3）用于表示节气、年龄.例如:

　　ChildrenoftenreceivepresentsatSpringFestival.

　　Jimbegantoplaythepianoattheageofseven.

　　2.in

　　1）用于表示在一段时间内.例如:

　　Motherwillbebackinafewdays.

　　2）用于表示在非特指的早晨、上／下午、夜晚.例如:

　　MyEnglishteacheroftengetsupbefore6o'clockinthemorning.

　　MygrandparentslikewatchingTVintheevening.

　　3）用于表示月份、季节、年、世纪等.例如:

　　IwasborninFebruary,1963.

　　Everythingbeginstogrowinspring.

　　TheOlympicGameswillbeheldinBeijingin2008.

　　Thestoryhappenedinthe20thcentury.

　　3.on

　　1）用于表示在具体某天或某一特定日子的上午、下午或晚上.例如:

　　HismotherwasbornonJanuary18,1956.

　　SheoftendoesherhomeworkonSaturdayafternoon.

　　Theoldmandiedinthemountainonacoldwinterafternoon.

　　2）用于表示节日.例如:

　　ChildrenoftengotochurchonChristmasDay.

　　PeoplevisittheirrelativesandfriendsonNewYear'sDay.

　　但是在含有earlymorning,lateafternoon之类的词语时,通常用in,而不用on.例如:

　　intheearlymorningof21st,March;

　　inthelateafternoonof15th,September

　　另外,由last,next,this,that,these,those等词构成的表示时间的短语前面,通常不用任何介词.yesterday,today,tomorrow,tonight等词前面也通常不用介词.例如:

　　HeboughtaChinese-Englishdictionarythismorning.

　　TheytraveledtoBeijinglastsummer.

　　You'dbetterfinishyourhomeworktonight.