用反问句表示强调：不是……吗？

Chinese uses 不是……吗 to push someone toward an obvious fact. You're not asking for new info. You're saying, "Come on, this is your phone, right?" The 吗 makes it a question in shape, but the 不是……吗 frame makes it a challenge. So when you hear it, agree or explain. Don't treat it as a neutral yes/no question.

不是……吗 is a fixed mold. The 吗 is what turns the whole thing into a rhetorical push. Swap in 呢 and you get something like 你不是有钥匙呢？, which sounds broken or like a plain curious question. 呢 doesn't carry the "obviously you do" force. So keep 吗. If you want a real question, ask 你有钥匙吗？

Without 吗, 这不是你的包 is a flat negative statement: "This is not your bag." The 吗 is the switch that turns that denial into a challenge. So the combo is not double-marking. 不是 gives the negative hook; 吗 makes it a rhetorical question. Remove either piece and the whole effect collapses.

The pattern pushes the listener to agree. That's fine with friends. With a boss, it can sound like you're calling them out. 你不是说今天放假吗？ means "Didn't you say today was a holiday?" The implied second half is "So why are you asking me to work?" If you want to be safe, use a softer opener like 我记得你说过今天放假，是不是？ instead.