1. at接时间点；in接一段时间；on接具体的时间；by不迟于，在……前。
at 10：00, at noon, at midnight, at sunrise；
on September 2nd, on the morning of May 1st；
I'll be back in three hours.
2. in和after后都可以接时间段，表示“在……之后”，但in常与将来时连用，after常与过去时连用。
We will meet again in two weeks.
They finished the task after two days.
