in ,on,at 在……時
in表示較長時間,如世紀、朝代、時代、年、季節(jié)、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表示具體某一天及其早、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表示某一時刻或較短暫的時間,或泛指圣誕節(jié),復活節(jié)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 等詞之前一律不用介詞.如：We meet every day.節(jié)日用at.上面那個你仔細看啊