Yet, however: 却
副词 / adverbs
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
A，(Subject) + 却 + Verb / Adj
却 (què) marks contrast or an unexpected result: "yet, but, however".
It's an adverb: it goes after the subject, before the verb, never at the start of a sentence (unlike 但是, which is a conjunction).
You can combine 但是 and 却 for extra emphasis: 我很早出门，但是却迟到了。
Examples
我们订了两个房间，酒店却只给了一个。
Wǒmen dìng le liǎng ge fángjiān, jiǔdiàn què zhǐ gěi le yí ge.We booked two rooms, yet the hotel only gave us one.
天气预报说晴天，却下了一整天雨。
Tiānqì yùbào shuō qíngtiān, què xià le yì zhěng tiān yǔ.The forecast said sunny, but it rained all day.
他去过很多国家，却从来没来过中国。
Tā qù guo hěn duō guójiā, què cónglái méi lái guo Zhōngguó.He's been to many countries, yet never to China.
Common mistakes
✗ 却我没去。 → 却 comes after the subject. ✓ 我却没去。
To start a sentence, use 但是 / 可是 instead.