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.)
三樓說的有道理,其實(shí)我各人覺得時態(tài)并不是很難的東西,只是要掌握它必須在初學(xué)時看到一個句子就想想為什么作者用這個時態(tài),想多了,自然就理解了.另外在賭局子時一定要注意該句子所發(fā)生的時間.