Both English speakers learning Chinese and Chinese learning English often have trouble with tense and aspect. To make things confusing for those learning English, verbs are marked for both tense and aspect, and the two are often conflated in meaning, such as "I ate [last night]" (occurred in the past: tense) and "I have eaten [already, now]" (activity has reached completion: aspect). In Mandarin, only aspect is marked on the verb (with 'le' 了 and other particles), and tense is not marked grammatically. To make things more confusing in Chinese, there is a modal particle 'le' 了 with a rather different meaning.
Students are often confused with these, so rather than give more explanations, I have collected several example sentences in which past tense must be marked in English but 了 should not be used in Chinese, as well as sentences with 了 in Chinese but no past tense marker in English.
In examples 1–15, the verb phrases in red happened in the past, but were not "completed" (or are actions that don't have an endpoint). They are thus marked as past tense in English, but are not marked with an aspect marker (i.e. perfective 'le' 了) in Chinese. The sentences contain a second part "...when V" to give context; such context is required in Chinese for a 'past' interpretation of the red phrases, since tense is not marked. Notice the commonalities in meaning of the red phrases.
1. She was tired
when she got home yesterday evening. 他昨天晚上回到家的时候已经很累
2. I was taking a shower
when you called. 你打电话来的时候我正在洗澡
3. I met her when I was taking a walk in the park
4. They had already left when I got there
5. They were here
only a few minutes ago. 几分钟前他们还在这里
6. Liyou won the award. She worked very hard
[last year]. 李友获奖了。[去年]她很用功
7. (After eating a large meal): I was hungry
8. I was tired
yesterday. Today I feel better. 我昨天觉得很累
9. There was a restaurant
here years ago. 几年前这里有一家餐厅
10. Very few girls went to school
11. He smoked
at that time, but he doesn't now. 当时他抽烟
12. Nobody liked dancing
back then. 那时代没有人喜欢跳舞
13. Is he really that young? I didn't know
14. I thought you were American
15. I haven't seen her again since she moved to China
In examples 16–20, the verb phrases happened in the past, and have also been completed, so English requires marking past tense, and Chinese requires marking for completed (perfective) aspect with 了 (or 没 in the negative). 16－21在过去时发生的，而是已经完成的。
16. I became a soldier (and still am). 我当了兵。
17. He watched three ballgames this week. 他这个星期看了三场球赛。
18. He drank that cup of tea. 他喝了那杯茶。
19. I have eaten. 我吃饭了。
20. He didn't come to eat. 他没来吃饭。
In 21-25, the red phrases occur in the present or future, but have been completed. Past tense is not marked in English, but perfective aspect is marked in Chinese with 了.
21. Let's talk about it when the movie is over
22. The contract is up
, I'm not working any more. 合同已经期满了
23. We can open it when we find the key
the test over
25. We don't have to be nervous any more once we're done writing the thesis
Examples 26-28 contain 了 in Chinese，but it marks inceptive mood, not completed aspect.
26. I don't have any money left (now). 我没有钱了。(inceptive mood)
27. It's raining. 下雨了。
28. You're coming, great! 你要来了，太好了！
Finally, here are two examples with perfective aspect that are difficult to translate to English, but I think they really help to illustrate a key difference between English and Chinese: perfective aspect with 了 in Mandarin marks actions that have ceased, but have not necessarily reached their "natural" endpoint as is assumed in English.
29. I started writing a letter yesterday, but didn't finish. 我昨天写了一封信，可是没写完。
(lit. I wrote a letter yesterday, but didn't finish - incorrect in English because "wrote a letter" implies that it was finished)
30. She picked [at] the apple, but didn't pick it [off the tree]. 她摘了苹果，但是没有摘下。
To me, this shows that simple verbs in Chinese (like 写 and 摘) don't imply completion and termination of an action the way English verbs like 'pick (an apple)' do; completion must be explicitly marked by words like 了, 完, 下 etc. This is somewhat similar to how tense must be explicitly marked in English.
If there are any questions or corrections, please leave a comment below!