1. Explain it simply
Put 不是 right before the predicate and 吗 at the very end of the sentence.
It is not a real yes-or-no question; it nudges the listener to agree with something you both know.
你不是老师吗？really means 'You are a teacher, right?' rather than genuinely asking.
2. Show examples
你不是老师吗？
Nǐ bù shì lǎoshī ma?
Sandhi:Nǐ bú shì lǎoshī ma?
Aren't you a teacher?
confirming a fact
不是说今天下雪吗？
Bù shì shuō jīntiān xià xuě ma?
Sandhi:Bú shì shuō jīntiān xià xuě ma?
Didn't they say it would snow today?
不是说 = didn't they say
她不是你姐姐吗？
Tā bù shì nǐ jiějie ma?
Sandhi:Tā bú shì nǐ jiějie ma?
Isn't she your older sister?
mild surprise
你不是喜欢喝茶吗？
Nǐ bù shì xǐhuan hē chá ma?
Sandhi:Nǐ bú shì xǐhuan hē chá ma?
Don't you like drinking tea?
不是 before a verb phrase
3. Share a tip
Keep 不是 as one block in front of the predicate and always end with 吗 - if you split them or move 吗 into the middle, the sentence breaks.