Chinese does not really have past tense. Instead they have 了, 过 and the 是。。。的 pattern. Oh and 已经 which means already. 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!

That's not correct. And it's too much to elaborate on this particle. There really is a lot to learn.

Judging by the answers given, 了 is added to the end of a verb to indicated that it had been done, correct?

不好了。 怎么了？

The exceptions 1) 快了, 快了 (means I am coming/ I am going to finish it./Almost) Conversation: 1)A:你好了沒?? (Are you ready or not?) B: 快了, 快了!(Almost!) 2)A: 你到了沒? (Did you arrive ?) B: 快了 ,快了(Almost, I am coming.)