1. Explain it simply
First reject the wrong assumption with 不是 A, then give the correct answer with 而是 B.
Order: 不是 + rejected point，而是 + true point. Both markers are needed and keep this order.
The two parts are contrasted, not both true - A is denied, B is affirmed.
而是 must stay in the second clause; do not drop it or replace the first 不是 with plain 是.
2. Show examples
他不是老师，而是医生。
Tā bù shì lǎoshī, ér shì yīshēng.
Sandhi:Tā bú shì lǎoshī, ér shì yīshēng.
He is not a teacher, but a doctor.
不是 rejects, 而是 gives the truth
这不是茶，而是咖啡。
Zhè bù shì chá, ér shì kāfēi.
Sandhi:Zhè bú shì chá, ér shì kāfēi.
This is not tea, but coffee.
A is denied, B is affirmed
他去的不是学校，而是医院。
Tā qù de bù shì xuéxiào, ér shì yīyuàn.
Sandhi:Tā qù de bú shì xuéxiào, ér shì yīyuàn.
Where he went is not the school, but the hospital.
both markers stay in order
3. Share a tip
Keep both halves - 不是 for the wrong idea and 而是 for the right one; dropping 而是 leaves the correction unfinished.