Chinese does not really have past tense. Instead they have 了, 过 and the 是。。。的 pattern.
了 It is to show a completed action/an accomplished fact.. 我做完了-I have done it-I have finished.
It is important to understand it is not showing past tense, but completion because I could say things like 你得写了字， 才能去玩儿-You have to finish writing characters, then you can go play.
了 is also attached to the end of certain verbs such as forget, kill, sell etc. ? For example 别忘了! Don't forget!
过 is to show an something you have experienced in the past-我去过中国-I have been to China. 我吃过中国菜。I have eaten Chinese food.
是。。的 is harder to explain. This pattern is used to highlight the circumstances of past events. It's for when you want to get the answers to things like where, when, how etc, or when you want to highlight those details.你在哪里看到她的？Where did you see her？我在商店看到她的-I saw her at the shop. 你是怎么来的？-How did you get here？是飞机来的- I came by plane
Note: You can omit 是 in the affirmative, but not in the negative, hence why sometimes I left out 是)
Hope this helps!