现在进行时与与过去时？

http://baike.baidu.com/view/201163.html?wtp=tt

这个是现在进行时

http://baike.baidu.com/view/901109.html?wtp=tt

这个是过去时

sb be doing sth.进行时
sb did sth.过去时
例句:
I am doing my homework now.进行时,我正在做作业。通常有now等表示动作正在进行的词语，用进行时。
I finished my homework yesterday.过去时，我昨天完成了我的作业。通常有yesterday等表明过去时态的词时，用过去时。

be doing sth. 就是正在发生的事情
eg. Mr Li is washing his car.
sb./sth.did sth.以前做的事 现在不做了
eg. XiaoWang went to school on foot yesterday.

be doing eg: I am doing my homework.
did or was eg: He went to Beijing.