动态助词：了

The aspect particle 了 (动态助词了) attaches to a verb to indicate that the action has reached completion. Think of it as a 'done' stamp on the verb, not a past-tense marker.

When to use 了: (1) To report a completed action — 我买了一本书 (I bought a book). (2) When a specific amount or object follows — 他吃了两个苹果 (He ate two apples). (3) To describe a sequence of completed events — 我回家了，吃了饭，然后睡觉了。

When NOT to use 了: (1) Habitual/routine past — do NOT say 我以前每天了喝咖啡; instead say 我以前每天喝咖啡. (2) States or conditions — do NOT add 了 to adjectives or stative verbs like 知道, 喜欢. (3) After negation — use 没/没有 and remove 了: 我没买书, NOT 我不买了书.

Common mistake: Learners often add 了 to every verb when talking about the past. Only add 了 when there is a clear completed, bounded action — not for general past descriptions.

Note: Sentence-final 了 (语气助词了) is different — it marks a change of state or a new situation (下课了 = class is over now). This lesson focuses on verb-suffix 了 only.