Context. It’s all about context.
I teaching, contextualize the useage and the phrase, and emphazise in the right place at right time.
People her think that if they can master slang, they are cool and have a high level of competence, hence, they like to show it off. That is why it is included in a textbook, to show off. Just as when you are told when translating to use complicated words so people will know they are getting their money’s worth even if they do not get the meaning of what you wrote.
Present examples from movies and songs and explain why this contraction is used. THEN present the dead data from the textbook. You decide if you want to point out any mistakes -and find a new job- or let it go and make it part of an exercise to determine whether the examples are correct.
Have the students do a little dramatization and encouraneg the fun part of the colloquial context. Hopefully by then they will be able to tell the difference.
I was taught this way, and still it was trial and error with stuff like “ain’t” which my cousins used, but as a joke or in certain contexts, and took me a while to discover that the joke was on me. I taught it later on in context, with songs, or if it came up in a video we were watching. Most of the time, people already know these phrases from the movies. What they do not know is why they are there.