不…才怪 / …才怪【bù…cái guài / …cái guài】

X 才怪 means "it would be strange if X". With 不 in X, the sentence says the thing is sure to happen: 他不迟到才怪 is "it'd be strange if he weren't late", so he will be. 呢 often follows. It's spoken and a little mocking.

Without 不: definitely not

Leave out 不 and the meaning flips: the thing named won't happen.

才怪 or 才怪呢 also stands alone as a reply: "yeah, right".

Getting the direction right

The two forms say opposite things, and learners reverse them. Decide what you're sure of, then put its opposite before 才怪.

• × 他这么聪明，考得好才怪（meaning "he'll do well"）→ 他这么聪明，考不好才怪