but作为连接词（ conj.）时可以与逗号连用
1.但是
It is hot in summer here,but it is not cold in winter.
这里夏天热,但冬天不冷.
2.可是,然而
She looks very young,but she is already in her 30's.
她看上去很年轻,可是她已三十多岁了.
3.(表示惊讶、不同意等)喔,哇
Heavens,but it snows.
天哪,真的下雪了.