on,以下是辨析：
【in】我是“大姐”,因?yàn)槲液竺嫠拥亩际禽^長時(shí)間.具體用法有：
1． 表示在較長的時(shí)間里（如周／月份／季節(jié)／年份／世紀(jì)等）.如：in a week； in May； in spring／summer／autumn／winter； in 2008； in the 1990’s等.
2． 表示在上午、下午或晚上.如：in the morning／afternoon／evening.
3． in the daytime（在白天） 屬于固定搭配,指從日出到日落這一段時(shí)間,反義詞組是in the night.
4． “in ＋ 一段時(shí)間”表示“多久以后／以內(nèi)”,常與將來時(shí)連用.如：in half an hour； in ten minutes； in a few days等.
【on】我是 “二姐”,我后面所接的時(shí)間多與日期有關(guān).具體用法有：
1． 表示在具體的某一天（如日期、生日、節(jié)日或星期幾）.如：on May 4th,1919； on Monday； on Teachers’ Day； on my birthday； on that day等.
2． 表示某一天的上午、下午或晚上.如：on the morning of July 2； on Sunday afternoon； on a cold winter evening等.
【at】我是“小妹”,因?yàn)榻釉谖液竺娴臅r(shí)間最短.具體用法有：
1． 表示在某一具體時(shí)刻,即幾點(diǎn)幾分.如：at six o’clock； at half past nine； at a quarter to six； at this time等.
2． 表示在某一短暫的時(shí)間.如：at noon； at this moment； at the end of a year； at the start of the concert等.
用介詞at、on、in填空
1．He came to my family ＿＿＿＿＿＿＿ May,1999．
2．He got home ＿＿＿＿＿＿＿ midnight．
3．Tree leaves begin to fall ＿＿＿＿＿＿＿ September．
4．He went to Beijing ＿＿＿＿＿＿＿ the morning of October 8th．
5．This matter happened ＿＿＿＿＿＿ the 1970’s ＿＿＿＿＿＿ his eighties．
6．He usually goes to see a film ＿＿＿＿＿＿ Sunday．
7．Can you finish the task ＿＿＿＿＿＿＿ a month?
8．He left his homeland ＿＿＿＿＿＿＿ the age of thirty．
9．My family will move there ＿＿＿＿＿＿ two months．
10．He is kind to her ＿＿＿＿＿＿ times．