in+时间 表示在今后的多少时间之内.一般用在将来时,包括过去将来时,eg.I will finish this task in 3 days./ she said she would finish the task in 3 days.
after+时间 表示多少时间以后.不管是过去式现在时还是别的时态,只要符合句意都可以用.eg.i will finish this task after 3 days./i finished this task after 3 days.