不是…而是【bú shì … ér shì】

不是 A 而是 B says A is the wrong idea and B is the right one. A and B are the same kind of thing – two nouns, two verb phrases, two reasons – so the second clause directly replaces the first.

It is neutral to formal and sounds deliberate. In casual speech 而 often drops: 他不是老师，是医生.

Correcting a reason

The most common use is putting a reason right: not the cause people assume, but the real one.

Mixing the frames

不是…而是 pairs with nothing else. 而且 adds a point; 而是 replaces one.

• × 他不是老师，而且是医生 → 他不是老师，而是医生
• × 不但…而是 → 不但…而且

不是 A 就是 B looks similar but means "if not A, then B": either one or the other is true. It has its own point.