一般过去时:表示过去曾发生过的事,与现在无关,
谓语动词用过去式.
She went to the country last week.
现在完成时:表示一件所做过的事的结果对目前的影响,那事可能还要做下去,也可能才结束.
谓语用:have/has+动词的过去分词(pp).
I have lived in Shanghai for four years.
She has finished her homework.
过去完成时:表示过去某个时间点前已经做完的某件事.就是过去的过去
谓语用:had + PP
The bus had left when I got there.