It 's very difficult to explain it in english. Because they have many meaning in chinese. But if you only ask past tense. I try my best to explain it . 的 ：use it behind the verb eg ：他昨天买的票。 He bought the ticket yesterday. 了： use it behind the verb and adj , means it have been done eg ： 吃完了 。 I have eaten it. In past tense , people use 了 , but may be there are some special verb use 的 . Give some examples , we will help you . My english is not good enough . XD .

could u please give some examples u confused ?*^_^*

It's too complicated.一句两句说不清楚。而且这两个字有很多种用法。最好找个现实中的中国人来帮你解释。GOOD LUCK!