1. "了" Indicates a Change of State

   Comparison with English: English uses the simple present tense but implies a "change." Chinese explicitly uses "了" to mark this change.

• 我累了。 (I am tired now.) -> I wasn't tired before, but my state has changed.

• 他现在是老师了。 (He is a teacher now.) -> He wasn't a teacher before.

• 天气冷了。 (The weather is cold now.) -> The weather has turned cold.

• This is one of the most basic and important functions of "了". It signals that a situation has changed.

• Structure: [Situation] + 了

• Examples:

2. "了" Indicates Completion of an Action

   Important Note: If the sentence has a clear time adverb (like "昨天" - yesterday, "每天" - every day) and describes a habitual or regularly occurring past action, you cannot use "了".

• Incorrect: 我去年每天去了健身房。

• Correct: 我去年每天去健身房。

• 我昨天看了一场电影。 (I watched a movie yesterday.)

• 他买了三个苹果。 (He bought three apples.)

• This usage is closer to the English past tense, but it emphasizes the "completion" or "realization" of an action, rather than just that it happened in the past.

• Structure: [Subject] + [Verb] + 了 + ([Object])

• Examples:

Summary: When you want to say "the situation has changed" or "the action is completed," think of "了". Don't mechanically equate it with "-ed".