反问句"不是……吗？"

"不是……吗？"是反问句，说话人心里已经有答案，用这个句子来强调，或者表示有点儿奇怪、想让对方想起一件事。意思是"……是这样的"。

不是……吗? looks like a question, but the speaker already believes the answer is "yes". It means "Isn't it true that...?" and is used to remind someone of something, to stress a point, or to show surprise when things don't match what you expected: 你不是有考试吗? "Don't you have an exam (so why are you here)?" Put 不是 before the part you want to stress and 吗 at the end.