1、“would have done ”用于说明“以前本来要做某事,而现实是没有做”,eg：If you didn't call me ,i would have finished my work.如果你没打电话给我,我本来可以做完工作的.
2、一般过去式：就是简单地用于陈述过去的事情,是最基本的过去时态 eg:I went to park yesterday. 我昨天去公园了.
3、过去完成式：顾名思义,是说明过去已经做完的,而对现在又有影响的事.eg: I have prepared this project since two months ago ,so that it can be so successful. 我从2个月前就开始准备这个案子,所以它才可以这么成功.
希望有帮助到你,具体的最好系统的看下语法书,可以举一反三.