1. Explain it simply
难道 forms an emphatic rhetorical question. It does not ask for real information; it shows the speaker's surprise or strong feeling.
The expected answer is the opposite of what the question says: 难道你不明白吗 implies 'you really should understand'.
Word order: 难道 + subject + verb phrase + 吗？ 难道 can also come right after the subject: 你难道...吗？
It often pairs with 不 and fits a yes-no question that ends in 吗.
2. Show examples
难道你不认识他吗？
Nándào nǐ bù rènshi tā ma?
Sandhi:Nándào nǐ bú rènshi tā ma?
Could it really be that you don't know him?
implies you really should know him
难道他真的生病了吗？
Nándào tā zhēn de shēngbìng le ma?
Could he really be sick?
surprise, hard to believe
难道这不是你的手机吗？
Nándào zhè bù shì nǐ de shǒujī ma?
Sandhi:Nándào zhè bú shì nǐ de shǒujī ma?
Could this really not be your phone?
expects the answer 'yes, it is'
3. Share a tip
难道 works best with 吗 at the end - keep the 吗 to signal the rhetorical question.