了

了 (le) is one of the two most challenging particles in Mandarin because it covers two related but distinct grammatical jobs, and neither maps cleanly to English. First, 了 after a verb marks COMPLETED action: 我吃了一个苹果 (I ate an apple — the eating is finished). It is NOT 'past tense' — Chinese has no tense — it marks the verb as a completed event regardless of when it happened. Second, 了 at the end of a sentence marks a CHANGE OF STATE: 下雨了 (it started raining — wasn't raining before, is now), 我会说中文了 (I can speak Chinese now — couldn't before). Many sentences need both 了s. Learners get burned by overusing it: do not put 了 on habitual actions (我每天去 'I go every day' — no 了), on stative verbs of feeling (我喜欢, not 我喜欢了), or on most negative sentences (use 没 instead of 不…了 for past negation). Sentence-final 了 also gets attached to numbers to mean 'already': 三十了 ('already thirty').

Note: Neutral and universal — used in every register, but absolutely required in casual speech. Skipping it where it's needed sounds 'foreign'.

Memory hook: Two jobs: after a verb → 'did' (completed). At the end of a sentence → 'now' (new state).