了 (le) — completed action and change of state
Mandarin verbs don't conjugate for tense, so how do you say 'I ate' instead of 'I eat'? You add the particle 了. 我吃了一个苹果。 = I ate an apple. But 了 isn't strictly a 'past tense' marker — it does two distinct jobs depending on where it sits. Right after the verb, it marks the action as completed (often, but not always, in the past). At the end of the sentence, it signals a change of state — 'now things are different'. The two positions can even appear in the same sentence with different meanings. The huge trap: don't negate a past action with 不. Use 没 instead, and the 了 disappears.
了 after the verb — completed action
Pattern: subject + verb + 了 + object. 我吃了饭。 = I ate. 她买了一本书。 = She bought a book. 我们看了一个电影。 = We watched a movie. The action is finished — usually past, but sometimes a future completion: 我吃了饭就去。 = After I've eaten, I'll go. With a quantified object (a number + measure word, or 很多, 一点), this 了 is required and feels totally natural. Without quantification, a bare 我吃了饭 is fine but often gets a follow-up clause.
了 at sentence end — change of state
Drop 了 at the very end of a sentence and it means 'something has changed' or 'the situation is now X'. 我饿了。 = I'm hungry now (wasn't before). 下雨了。 = It's raining (now / it's started). 她是老师了。 = She's a teacher now. 我不去了。 = I'm not going (anymore). Translate with words like 'now', 'already', 'anymore', or just a tense shift in English. This 了 carries no past tense by itself — 下雨了 could be 'it has started raining' OR 'now it's raining'. The point is the change.
Both at once — verbal 了 + sentence 了
Sometimes you'll see two 了s in different slots: 我吃了三个苹果了。 = I've already eaten three apples (and might eat more, or this is the running total). The verbal 了 says the eating happened/completed; the sentence-final 了 adds the 'as of now / so far' flavor. You don't have to produce these yet — just recognize the pattern.
Negation: 没 instead of 不, and 了 drops
To say 'I didn't eat', you use 没 (or 没有) BEFORE the verb — and the 了 disappears. 我吃了饭。 → 我没吃饭。 = I didn't eat. 她买了书。 → 她没买书。 = She didn't buy a book. 我们看了电影。 → 我们没看电影。 = We didn't watch a movie. NEVER 我不吃了 for 'I didn't eat' — that means 'I'm not eating anymore' (sentence-final 了, change of state). The split is sharp: 没 + verb = past negation. 不 + verb = present/general negation.
Time phrases pair naturally with 了
Time-when words from lesson 7 — 昨天 (yesterday), 今天 (today), 去年 (last year) — sit at the front or right after the subject, just like before. 昨天我吃了一个苹果。 = Yesterday I ate an apple. 她去年去了中国。 = She went to China last year. The 了 is still on the verb; the time word just anchors when. With a frequency or duration phrase, 了 is also expected: 我学了三年中文。 = I've studied Chinese for three years.
