The paired-conjunction structure
English uses "although" OR "but", never both. Chinese comfortably uses both: 虽然这家店贵,但是菜很好吃 (although this restaurant is expensive, the food is good). Dropping 虽然 leaves a plain "but" sentence: 这家店贵,但是菜很好吃. Dropping 但是 leaves the concession dangling: 虽然这家店贵,菜很好吃 (this works but feels incomplete to native ears). Formal style prefers both together; casual speech often keeps just 但是.
可是 and 不过 as 但是 alternatives
可是 (kěshì) is interchangeable with 但是 in speech, slightly more casual. 不过 (búguò) is a softer "but" that downgrades the contrast, closer to English "however" or "that said". 虽然...可是 and 虽然...不过 are both valid variations. 但是 is the most formal and clear; pick it for writing, presentations, and anything that needs clarity.
这个方案虽然成本高,不过效果最好。
Zhè ge fāng'àn suīrán chéngběn gāo, búguò xiàoguǒ zuì hǎo.
This plan is costly, but the effect is best.
Position: concession first, contrast second
The default order is 虽然 + concession clause, 但是 + contrast clause. 虽然我累,但是我要去 (although I am tired, I am going). Flipping is grammatical but changes the emphasis and drops 虽然: 我要去,但是我累 (I am going, but I am tired). The flipped version foregrounds the main claim and trails the concession, common in spoken Chinese when reacting to a question.
虽然 as a single-word concession
In short spoken responses, 虽然 alone can mark a concession without the full clause pair: 虽然这样 (although it is so), pause, continue. This fragment use is informal and carries the feeling of the speaker thinking aloud. In writing or careful speech, use the full 虽然...但是 pattern.
虽然他年轻,但是经验很丰富。
Suīrán tā niánqīng, dànshì jīngyàn hěn fēngfù.
Although he is young, his experience is very rich.