了: 1. You can put it after a "verb", but "了" is meaningless in this situation, except for emphasizing the action. ex. 我喝了一杯茶 (I drink a cup of tea.) 我打了通電話 (I make a phone call.) 2. Put it at the end of a sentence. It includes the information of something done already. ex. 我回家了 (I'm home already) 我吃饱了 (I'm full already) 的: Usually connected with "adj." and put before a noun which is going to be describe.. ex. 漂亮的衣服 (beautiful clothes) The above are common usage of "了" and "的". Reply me if you have any question : )

In short, eat=Chi (吃)；ate=Chi le(吃了) You can never change the form of Chinese character in order to mean the tense. But you can add a 了 after a verb. My book=a book of mine=wo de shu（我的书）