一．介词at/ in /on .
1.表示时间：
1).表示某一具体时间点，某一时刻/ 年龄
at six o’clock at noon at that time
at the moment at the age of at night
2)in表示时间段,一天的三个时间段以及月份,年,季节,世纪,人生的某个时期(某人几十岁时)
in the morning/afternoon /evening
in spring /in 2007/in March
in the twenty-first century
in his fifties
