in加时间段,用于将来时,表示将来一段时间后.而after加时间段,用于过去时,表示从过去某个时间点算起一段时间后.另外,after还可接时间点,既可用于将来时也可用于过去时.例：
He will be back in two days.(或in two days' time)他两天后会回来的.
He came back after two days.他两天后回来了.
He came back after two o'clock.他两点后回来了.
He will come back after two o'clock.他两点后会回来的.