Look, the building is falling down.
快看，那栋楼房倒塌了。

You are always changing your mind.
你老是改变主意。

Why did you get up so early?
你怎么起得这么早？（强调起床的动作发生过了。）
Who hasn't handed in his paper?
有谁还没交试卷？（强调有卷子未交，可能造成不公平竞争。）
I saw this film yesterday.
我昨天看了这部电影。（强调看的动作发生过了。）
I have seen this film.
我看过这部电影了。（强调对现在的影响，电影的内容已经知道了。）
She returned yesterday.
她昨天回来了。（强调回来的动作发生过了。）
She has returned from Paris.
她已从巴黎回来了。（强调对现在的影响，已经可以找到她本人了。）