The structure 不是…吗？
“Isn't it?, "Don't you?”
The structure 不是...吗？ 
is used to ask a rhetorical question.
is used to ask a rhetorical question.
Pay attention
Rhetorical questions are questions where an answer is not really expected. It is often used to express indignation or astonishment.
Scheme
subject
不是
predicate
吗?
不是 can be followed by different parts of a sentence.
If a predicate is expressed by the verb 是 , there is no need to state it a second time.
, there is no need to state it a second time.Examples
我不是已跟你说了吗？怎么又问？
Haven't I told you that already? Why are you asking again?
wǒ bú shì yǐ gēn nǐ shuō le ma? zěnme yòu wèn
老师不是中国人吗？为什么不知道怎么用筷子？
Isn't the teacher Chinese? Why doesn't he know how to use chopsticks?
lǎoshī bú shì zhōngguórén ma, wèishénme bù zhīdào zěnme yòng kuàizi
你不是她的哥哥吗？
Aren't you her brother?
nǐ bú shì tā de gēge ma
Let's compare the last example with an interrogative sentence:
你不是她的哥哥吗？— Aren't you her brother?
The speaker knows that the person he/she is talking to is really her brother. A rhetorical question emphasizes the obviousness of this information.
你是她的哥哥吗？— Are you her brother?
The speaker doesn't know whether the person he/she is talking to is her brother.
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