The “虽然……，但是……” pattern
The pattern “虽然……，但是……” (suīrán..., dànshì...) joins two clauses that contrast with each other.
虽然although
但是but
虽然 and 但是 may be used together, or just one of them.
Basic structure
虽然 + clause 1，但是 + clause 2
Meaning:
Although …, …
or
Even though …, …
Examples
虽然你忘了，但是我记得。
Even though you forgot, I remembered.
虽然 + 你忘了
但是 + 我记得
虽然although
你忘了you forgot
但是but
我记得I remember
The sentence sets two opposites side by side:
你忘了 — you forgot
but:
我记得 — I remember
外边下雪了，但是不太冷。
It's snowing outside, but it isn't very cold.
外边下雪了 + 但是 + 不太冷
it's snowing outside + but + not very cold
外边outside
下雪了it has started snowing
但是but
不太冷not very cold
Only 但是 is used here.
虽然觉得有点儿累，我还是走回家了。
Although I felt a bit tired, I still walked home.
虽然 + 觉得有点儿累
我还是 + 走回家了
虽然although
觉得有点儿累feel a bit tired
我还是I still
走回家了walked home
Only 虽然 is used here; 但是 is dropped.
Quick rule
Both: 虽然 + A，但是 + B
Although A, B.
虽然 alone: 虽然 + A，B
Although A, B.
但是 alone: A，但是 + B
A, but B.
虽然你忘了，但是我记得。— Even though you forgot, I remembered.
外边下雪了，但是不太冷。— It's snowing outside, but it isn't very cold.
虽然觉得有点儿累，我还是走回家了。— Although I felt a bit tired, I still walked home.