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.