in：过……后（未来时间）
注意
如果用于过去时,用after +时间.
She went to Nanjing last May,and she came back after a month.
去年五月她去了南京,一个月之后她又回来了.
大多用在将来时（一般将来时和过去将来时）.
in an hour 一小时之后
in a week or so 一个多星期之后
He will be back in five hours.他五小时之后回来.

They said they would arrive here in a week.他们说他们一周后到达.

within：不超过……的范围
比较
within和in with in强调在……时间之内,没有时态的限制.
in是以现在为基准,in an hour是指从现在起1小时之后,所以一般只用于将来时.
within 3 hours 3小时之内
I must finish painting the cat within five minutes.我必须在五分钟之内画好这只猫.
They worked hard.They finished the work
within 2 days at last.他们努力工作,结果终于他们在两天之内完成了这项工作.