不是 + Clause + 吗?
When 不是 appears at the beginning of a sentence, it can introduce a rhetorical question that implies the speaker already knows the answer. The speaker uses this to remind the listener of a known fact or to gently challenge a statement. It carries a tone of "isn't it the case that…?"
不是说好了今天一起吃饭吗?
bú shì shuō hǎo le jīntiān yìqǐ chī fàn ma?
Didn't we agree to have dinner together today?
不是你自己答应的吗,怎么又反悔了?
bú shì nǐ zìjǐ dāyìng de ma, zěnme yòu fǎnhuǐ le?
Didn't you agree to it yourself? How can you go back on your word?
不是还有时间吗,别着急。
bú shì hái yǒu shíjiān ma, bié zhāojí.
Isn't there still time? Don't rush.