了 for change: “now it's…”
【一22】【一31】【一40】语气助词“了” / 变化态 / 非主谓句
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
At the end of a sentence, 了 (le) signals a change of situation: something is new compared to before.
下雨了！ (It's started raining!): it wasn't raining before.
我饿了 (I'm hungry now). 她病了 (she's fallen ill). 他二十岁了 (he's twenty now).
With a negative, 了 signals a change of plan: 我不去了 (I'm not going after all).
To deny a change, use 没 without 了: 她没病 (she isn't sick). 雨没小 (the rain hasn't eased).
Some sentences don't even have a subject: 下雨了。 That's normal in Chinese for weather.
Examples
下雨了！
Xià yǔ le!It's raining!
我累了。
Wǒ lèi le.I'm tired.
她病了。
Tā bìng le.She's fallen ill.
雨小了。
Yǔ xiǎo le.The rain has eased.
我不去了。
Wǒ bú qù le.I'm not going after all.
Common mistakes
✗ 我饿 (sounds incomplete) → ✓ 我饿了 / 我很饿.
✗ 她没病了 → ✓ 她没病.
了 ≠ past tense: 下雨了 describes the current situation (it's raining now).