虽然…但是…: although… (but)
复句 / complex sentences (转折)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
虽然 + A，但是 / 可是 + B
To express a concession, Chinese uses a pair: 虽然 in the first clause, 但是 (or 可是) in the second.
In English, "Although it's hard, I like it" has no "but". In Chinese you keep both: 虽然很难，但是我很喜欢. It looks redundant, but it's the norm.
虽然 can go before or after the subject of the first clause.
Examples
虽然汉字很难，但是很有意思。
Suīrán Hànzì hěn nán, dànshì hěn yǒu yìsi.Although characters are hard, they're really interesting.
他虽然很忙，但是每天都学习一个小时。
Tā suīrán hěn máng, dànshì měi tiān dōu xuéxí yí ge xiǎoshí.Although he's very busy, he studies an hour every day.
虽然考得不太好，可是我不难过。
Suīrán kǎo de bú tài hǎo, kěshì wǒ bù nánguò.Although the exam didn't go that well, I'm not upset.
这本书虽然很旧，但是很有用。
Zhè běn shū suīrán hěn jiù, dànshì hěn yǒuyòng.This book is old but very useful.
Common mistakes
No comma right after 但是 is needed: ✓ 但是我很喜欢.
✗ 虽然…所以…: 虽然 pairs with 但是/可是, not 所以.