at: 用于表示时刻，时间的某一点
at noon, at night, at present
on: 用于星期， 某天， 某一天的上午、下午、晚上（指具体的某一天时，一律用on)
on Sunday, on Sunday morning, on March 8
in: 用于表示周，月，季节，年，泛指上午、下午
in 1999, in November, in summer, in the afternoon
at, on, in 范围由小变大， 除了in还用于上下午
by: 在……前（时间）， 截止到……
By the time I arrived, she had already gone.
for, during, through, from
since: 自从……以来（表示从以前某事一直到现在仍在继续）
Since that time, my eyes had never been very good.
within：不超过……的范围
He will arrive within an hour.
