Why is the saying incorrect? Don't fix it if it ain't broke?

Yes, 'broken' would be the better word, but 'broke' is accepted as the past tense of 'break' despite being incorrect. "Ain't" is another accepted word that is incorrect.

"Don't fix it if it ain't broke" is just an expression, a saying. It isn't supposed to be correct. It means to leave things alone. It really has nothing to do with fixing things or not fixing things! Have you heard someone say to "leave well enough alone?" That is what it means. Unless there is a problem, leave it be. Even if you don't think it is right, sometimes it is better not to try to "fix" a problem! It makes things worse.

Try not to learn English, or American, expressions. They make little sense. These are idioms, expressions, or sayings only, not literal.

Literally, it is "Don't fix it if it isn't broken." But,again, this is an expression.

What it is not, is Ebonics! That is something else entirely, Kiki!

Yes, the phrase "don't fix it if it ain't broke" is gramatically incorrent. This sentence is what is known as Ebonics, which is just a form of speaking and writing butchered English language. The grammatically corrent way to say it is "don't fix it if it isn't broken."

A piece of advice; If you are trying to learn English correctly, stay away from Ebonics or any kind of street talk. It will just confuse you and cause you to speak and write the language incorrectly.

It's an idiom, some are grammatically incorrect but always have a little bit of wisdom in them.

"If it ain't broke, don't fix it," simply means, any attempt to improve on a system that already works is pointless, and could make it worse.