in 在...以后 (指时间)
He'll come back in a week.
他将于一周之后回来.
behind在后面 (指空间)
Tom sits behind me
after 在...以后 (时间先后)
After graduation he went abroad.
毕业后他去了国外.
At half after ten it began to rain.
在十点半下起雨来.