虽……，但 / 可 / 却 / 也……: written “although”
【六49】【六21】复句 / complex sentences
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
虽 is the short, written form of 虽然: “although, even though”. The second half gives the answer to that concession: 但 or 可 at the head of the clause, or 却 and 也 after the subject. 他年纪虽小，但经验不少; 他年纪虽小，经验却不少.
The four words do not sit in the same place. 但 and 可 open the clause (但 is more written, 可 livelier). 却 and 也 slip in after the subject, before the verb or adjective, never at the head. 也 means “all the same”: 她身体虽不舒服，也坚持把工作做完. On its own, 可 is also a plain “but”: 我们约好了一起去，可他忘记了.
Compare 虽然……但是…… (hsk2-g42), 尽管 (hsk3-g49), 却 (hsk3-g30) and the more bookish 虽说 (hsk6-g26). No 所以 or 因此 in this sentence: 虽 calls for a reversal, not a consequence.
Examples
他年纪虽小，经验却不少。
Tā niánjì suī xiǎo, jīngyàn què bù shǎo.He is young, yet he has plenty of experience.
这家公司规模虽小，但实力很强。
Zhè jiā gōngsī guīmó suī xiǎo, dàn shílì hěn qiáng.This company is small in scale, but it is very strong.
这款电池价格虽高，可质量确实好。
Zhè kuǎn diànchí jiàgé suī gāo, kě zhìliàng quèshí hǎo.This battery is pricey, but the quality really is good.
我们约好了一起去，可他忘记了。
Wǒmen yuē hǎo le yìqǐ qù, kě tā wàngjì le.We had agreed to go together, but he forgot.
她身体虽不舒服，也坚持把工作做完。
Tā shēntǐ suī bù shūfu, yě jiānchí bǎ gōngzuò zuò wán.Although she was unwell, she insisted on finishing her work.
谈判虽然进行得不太顺利，但双方都愿意继续沟通。
Tánpàn suīrán jìnxíng de bú tài shùnlì, dàn shuāngfāng dōu yuànyì jìxù gōutōng.The talks are not going very smoothly, but both sides are willing to keep communicating.
Common mistakes
✗ 他年纪虽小，却他经验很多 → ✓ 他年纪虽小，经验却很多 (却 and 也 come after the subject; only 但 and 可 open the clause).
✗ 虽然他年纪小，所以经验不少 → ✓ 虽然他年纪小，但经验不少 (虽 calls for a reversal, not a result).