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.如：We meet every day.节日用at.上面那个你仔细看啊