了 after a verb: a completed event
Verb-final 了 le marks a change or completed event in a particular situation. It does not simply mean “past tense”; the context tells you when the event happened.
Wǒ chī le fàn. — I ate / I have eaten.
Tā mǎi le yì běn shū. — He bought a book.
Xiàyǔ le. — It has started raining.
A sentence-final 了 can emphasize a new situation: 我懂了 means “Now I understand.” The same character can have related but different positions and functions.
过 for life experience
过 guo after a verb says that someone has had the experience at least once. It does not focus on a particular finished event or exact date.
- 你去过中国吗？ Have you ever been to China?
- 我看过这部电影。 I have seen this movie before.
- 我没吃过北京烤鸭。 I have never eaten Beijing roast duck.
在 before a verb: an action in progress
在 zài before a verb means that an action is happening now or around a particular time. 正在 makes the “in progress” meaning more explicit.
Wǒ zài xuéxí Zhōngwén. — I am studying Chinese.
Tāmen zhèngzài chīfàn. — They are eating.
Do not use 在 for every English present-tense verb. A general habit such as 我每天学习中文 (“I study Chinese every day”) does not need it.
Choose the perspective first
Ask what you want the listener to notice: a new completed event, a past experience, or an action unfolding now. That perspective is more useful than memorizing a direct English translation for each particle.
For negative sentences, remember that 没 commonly negates completed or experienced events: 我没去过 means “I have never been.” See the guide on 不 vs 没 for the larger contrast.