什么时候该用过去式,什么时候用现在完成时,什么时候又用过去完成时呢?

if you want to say a completed action in the past,or a habit in the past,you use simple past.eg.I saw a snake yesterday.I was good at maths when i was a child.
if something happened before another action in the past,you use past perfect.eg.I had bought a magzine before i got on the train.
if an action happened at an unspecified time before now,and the action cause some change or result ,you use present perfect.
eg.I have met him before.(so I know him.)
my French has improved since I came to France.(from poor to skillful.)
三楼说的有道理,其实我各人觉得时态并不是很难的东西,只是要掌握它必须在初学时看到一个句子就想想为什么作者用这个时态,想多了,自然就理解了.另外在赌局子时一定要注意该句子所发生的时间.