Frames a statement the speaker holds to be true as a negated question, pressing the listener to grant the obvious positi
框 · Frame
[subj] 不是 / 沒(有) [statement] 嗎?
觸 · Trigger
You want them to agree to something plain you both can see, so you ask it as a doubt you do not really hold.
序 · The move
1Hold the positive you want granted (it is good / she told you).Is this something the listener can already see is true?
2Pick the negator: 不是 for a present judgement, 沒(有) for a past event or possession.Past or 'have'? use 沒. A flat is-statement? use 不是.
3Set the negator in front of the statement and hang 嗎 on the tail.Does it now read as 'isn't it…?' rather than a real no?
4Read it back as the answer you expect.If the natural reply is 'yes, of course', the flip worked; if it invites a genuine 'no', it is an ordinary question.
Plain 嗎 wants the answer it does not yet know; 不是/沒…嗎 already holds the answer and presses you to agree. The front 不是/沒 is what flips a real question into a rhetorical one.
A-not-A (好不好) lays both yes and no open to pick from, neutral; 不是…嗎 has stacked the deck for yes before you answer.
✗ 你不知道嗎？(intended: surely you know) → ✓ 你不是知道嗎? (front 不是 makes it press the yes; bare 不知道嗎 can read as a genuine 'you don't know?')
✗ 沒看過嗎? for a past event you assume happened → ✓ 你不是看過了嗎? / 你沒看過嗎? (keep the 不是/沒 up front so the flip to 'you have, haven't you?' is clear; do not pair 沒 with 了)
✗ 這樣很好不是嗎 as the default → ✓ 這樣不是很好嗎 (the negator sits before the statement, 嗎 at the tail; the tag-style 不是嗎 is a separate, lighter move)
English speakers reach for a rising real question or a tag ('…, right?'). 不是/沒…嗎 is heavier: the negator goes in FRONT of the clause, not as a trailing tag, and the whole thing asserts rather than asks.