1) but表示转折，while表示对比。

Some people love cats, while others hate them.

典型例题

--- Would you like to come to dinner tonight?

--- I'd like to, ___ I'm too busy.

A. and　 B. so　 C. as　 D. but

答案D。but与前面形成转折，符合语意。而表并列的and， 结果的so，原因的as都不符合句意。

2) not…but… 意思为"不是……而是……"

not 和but 后面的用词要遵循一致原则。

They were not the bones of an animal, but (the bones) of a human being.