英语的过去时和普通话的时态很不一样的，都很难学的。请看下面30个列子。
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.
1-15里面红色的小句在过去时发生的，但是没有“完成”（或不是有终点线的动作），所以英文要用过去时态，但是中文不需要用‘了’。有错误请帮我纠正一下。
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. 我
正在公园散步的时候遇见了她。
4. They had already left when I
got there. 我
到那儿的时候他们已经走了。
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! (饱餐了一顿后)：我刚才
很饿!
8. I
was tired yesterday. Today I feel better. 我昨天
觉得很累。今天好了一点。
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 then. 那时几乎
没有女孩子去上学。
11. He
smoked at that time, but he doesn't now. 当时他
抽烟，但是现在不抽了。
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! 他真的那么年轻吗？我
不知道!
14. I
thought you were American. 我
以为你是美国人。
15. I haven't seen her again since she
moved to China. 自从她
赶移到中国之后我就未曾见过她。
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. 电影
看完了再说吧。
22. The contract
is up, I'm not working any more. 合同已经
期满了，我不工作了。
23. We can open it when we
find the key!
找到了钥匙就能打开了！
24.
Is the test
over? 考试
考完了没有？
25. We don't have to be nervous any more once we're
done writing the thesis. 论文
写完了我们就不用紧张了。
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!
