不是…吗
Rulebu shi ... ma?: a negative-question frame that asserts a strong positive ("isn't it ...? = it certainly is").
Curiosity
How does Chinese form a rhetorical question that looks like a question but is really a strong assertion, like "isn't this great!"?
Intuition
bu shi ... ma? asks in the negative to pull the listener into agreement. Literally "isn't it ...?" but the real meaning is "of course it is." Negative plus question stack up into a strong positive.
Visualization
Wrap the plain [this][is good] into [this][bu shi][good][ma]. bu shi goes in front, ma at the end, turning it rhetorical.
plain statement
这个subject很好adjective
rhetorical (bu shi…ma frame)
这个subject不是不是 (rhetorical frame)很好adjective吗吗 (question particle)
Essence
bu shi + (the point) + ma? asserts a strong positive. nandao ... ma? stresses surprise or rebuttal ("you don't mean ...?"). The form is a question but seeks no answer; it is a rhetorical device that stresses the speaker's assertion or feeling.
Examples
这个办法不是很好吗?
Isn't this a good method? (Of course it is.)
你不是已经答应了吗?
Didn't you already agree? (You clearly did.)
难道你不知道吗?
You don't mean you don't know? (You should.)
Mini-quiz
Which rhetorical question means "didn't I already say so?"