in +时间段：与将来时连用
after +时间段：与过去时连用
after +时间点：可与将来时连用
He will be back in two hours.
He came back after two hours.
He will be back after two o'clock.
in+一段时间表示将来时
如:I'll come back in a week
in＋一段时间也可用于过去时,意思是”．．．内”
如:he came in two days.
而after＋一段时间通常只能用于过去时,不能用于将来时
如：he came back after a week