The principle

Mandarin separates TIME (when) from ASPECT (what state the action is in). English fuses them into the verb form: 'I ate' carries both past time and completed aspect. Mandarin pulls them apart.

• Time goes at the FRONT of the sentence, in a time word: 今天, 昨天, 明天, 去年.
• Aspect goes AFTER the verb, in a particle: 了, 在, 过, 着.

The verb itself never changes. 吃 is always 吃.

了 (le): completion and new situation

The most-used, most-confused aspect particle. It does TWO related but distinct jobs.

Job 1: completion (post-verb 了)

When 了 sits directly AFTER the verb, it marks the action as completed.

• 我吃了饭。 (Wǒ chī le fàn.) - I ate / I have eaten.
• 他买了一本书。 (Tā mǎi le yì běn shū.) - He bought a book.
• 我看了那部电影。 (Wǒ kàn le nà bù diàn yǐng.) - I watched that film.

The action is finished. No commitment about when - that's the time word's job - just that it's done.

Job 2: new situation (sentence-final 了)

When 了 sits at the END of the sentence (after any object), it marks a CHANGE - the situation is new, or has reached a point that wasn't true before.

• 我吃饭了。 (Wǒ chī fàn le.) - I'm eating now / I've started eating. (NEW: I wasn't before.)
• 太晚了。 (Tài wǎn le.) - It's late now. (NEW: time has crossed a threshold.)
• 他来了。 (Tā lái le.) - He's here / He's come. (NEW: he wasn't before.)
• 我有钱了。 (Wǒ yǒu qián le.) - I have money now. (NEW: I didn't before.)
• 春天了。 (Chūn tiān le.) - It's spring now.

The clue is the change. 'Now I do', 'now it is', 'now I have' - the situation has shifted.

Both at once

A sentence can carry both functions of 了. They sit in different positions and do different jobs.

• 我吃了三个苹果了。 (Wǒ chī le sān ge píng guǒ le.) - I've eaten three apples (and the situation is that I've reached this point).

The first 了 is post-verb completion. The second is sentence-final change-of-state. Don't try to translate them separately; together they paint 'I've eaten three apples so far'.

When NOT to use 了

This is where English speakers over-apply 了. Three traps:

1. Habitual / general statements: 我每天吃饭, NOT 我每天吃了饭. Habit isn't completion.
2. Negative sentences: use 没 + verb (no 了). 我没吃 (I haven't eaten), NOT 我没吃了.
3. Stative descriptions: 'I am Chinese', 'he is tall', 'the book is on the table' - these aren't actions that complete. No 了.

在 / 正在: ongoing action

Pattern: subject + 在 (or 正在) + verb.

• 我在吃饭。 (Wǒ zài chī fàn.) - I'm eating.
• 他正在看书。 (Tā zhèng zài kàn shū.) - He's reading (right now).
• 我在学中文。 (Wǒ zài xué zhōng wén.) - I'm learning Chinese.
• 他正在工作。 (Tā zhèng zài gōng zuò.) - He's working (right at this moment).

在 by itself is the everyday form. 正在 adds 'at this very moment' emphasis. You can also end with 呢 for an even more casual ongoing feel: 我吃饭呢 (I'm eating, mate). All three say roughly the same thing.

Note that 在 here is the aspect marker, NOT the location-preposition 在 (at). Context disambiguates. 我在家 = I'm at home (location 在). 我在吃饭 = I'm eating (aspect 在).

过 (guo): 'have ever' experiential

Pattern: verb + 过. 'Have you ever done X?', 'I have done X at some point in my life.'

• 我去过中国。 (Wǒ qù guo zhōng guó.) - I've been to China (at some point).
• 你吃过火锅吗？ (Nǐ chī guo huǒ guō ma?) - Have you ever eaten hotpot?
• 我没看过这部电影。 (Wǒ méi kàn guo zhè bù diàn yǐng.) - I haven't seen this film.
• 他学过法语。 (Tā xué guo fǎ yǔ.) - He's studied French (at some point in the past).

The 过 says: at some unspecified point, the experience happened. It doesn't matter when, doesn't matter how recent, doesn't matter whether the result still holds. The experience is in your CV.

Negation of 过 uses 没 + verb + 过 (not 不). The 过 stays in. 我没去过中国 (I haven't been to China).

了 vs 过: the distinction

This trips up everyone. 了 says the action happened (often recently, often relevantly). 过 says the experience is in your past, at some unspecified point.

• 我去了中国。 - I went to China. (Specific occasion, often recent.)
• 我去过中国。 - I've been to China. (At some point. CV-like.)
• 我吃了饭。 - I ate (a specific meal, e.g. dinner just now).
• 我吃过那道菜。 - I've eaten that dish (before, at some point).

If you can swap in 'have ever' in English, use 过. If you mean a specific completed event, use 了.

Position summary

Time words (今天, 昨天, 明天 etc.) sit at the front, before the verb, after the subject. They establish WHEN; the aspect markers establish in what STATE.

What to drill

1. Stop mapping 了 to English past tense. 了 = completion or new situation. English past doesn't fit cleanly.
2. Negatives use 没, no 了. 我没吃, not 我没吃了.
3. 在/正在 for 'I'm doing X right now'. Position before the verb.
4. 过 for 'I've ever done X'. Position after the verb, negate with 没.
5. Time word + aspect particle = the full picture. 我昨天吃了 (yesterday + completion). 我以前吃过 (in the past + experiential).

For how the aspect system sits inside Mandarin's broader word-order machinery, see the word-order page.