表示时间的介词：
(1)in , on，at 在时
in表较长时间，世纪朝代时代、年、季节、月及一般（非特指）的早、中、晚
如：in the 20th century, in the 1950s, in 1989, in summer, in January,
in the morning, in the night, in ones life , in ones thirties
on表示具体某一天及某一天的早、中、晚。
如：on May 1st, on Monday, on New Years Day, on a cold night .
in January, on a fine morning, on Sunday afternoon.
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
在last, next, this, that, some, every 等词之前一律不用介词
如：We meet every day.
(2)in, after 在之后
in +段时间表示将来的一段时间以后；
after+段时间表示过去的一段时间以后；
after+将来点时间表示将来的某一时刻以后。
例句：My mother will come back in three or four days.
He arrived after five months.
She will appear after five oclock this afternoon.
(3)from, since 自从
from仅说明什么时候开始，不说明某动作或情况持续多久；
since表示某动作或情况持续至说话时刻，通常与完成时连用。
例句：He studied the piano from the age of three.
They have lived here since 1978.
(4)after, behind 在之后
after主要用于表示时间；
behind主要用于表示位置。
例句：We shall leave after lunch.
Lucy is hiding behind an old house.