but作為連接詞（ conj.）時(shí)可以與逗號連用
1.但是
It is hot in summer here,but it is not cold in winter.
這里夏天熱,但冬天不冷.
2.可是,然而
She looks very young,but she is already in her 30's.
她看上去很年輕,可是她已三十多歲了.
3.(表示驚訝、不同意等)喔,哇
Heavens,but it snows.
天哪,真的下雪了.