What is up with people saying " John and I's car?"

Language was not spelt the same way as it is today. I guess it is always evolving. My mom would go nuts if I said "them and I". She was a stickler for grammar.

we were posting at the same time I am a 'grammar nazi'

I don't understand why people get it so wrong.

to drop the other name out to see what sounds right to use,,,, then use it,,,,

so you would not say " I's car " you would say " my car " therefore it is
" John's and my car "

"me and John" did this or that, that they are overcompensating here...

Do we really not teach grammar anymore? I'm left wondering and yes, I freely admit that we all make the occasional error. Still...

It was so full of sentence fragments that I couldn't read it. It was annoying & often times, jarring. One of the writers in my critique group says that sentence fragments are a writing style, but I say they're incorrect grammar. An occasional sentence fragment to make a point is one thing, but sentence fragments throughout a manuscript is too much, IMO.

how to use "me" correctly tend to avoid using it at all.

They don't get it that "me" is correct when used as an object, and "I" is only for first person subject of a sentence. Even if they don't remember or understand the grammar rules, they can learn from example by listening to teachers give them examples.

A frequent mistake is "Between you and I." Between is a preposition. Therefore, the correct word is "me" as the object of the preposition.

Pronouns in general seem to be a problem for some people who can't distinguish between subject, object, or possessive. I hear people say, "us children" when children is the subject of the sentence so it should be "we children."

Or, as subjects of a sentence, "Her and I" instead of the correct "She and I."

But, as you said, we all make mistakes. I never did get "which" vs. "that" correctly.

Multiple times but always from 20-30 something's.

To me me and John isn't anywhere near as cringeworthy as John and I's

I’s. On tv and podcasts and radio.

I even looked it up one time because I thought maybe I missed something. I’s is not supposed to be a thing!

I also almost wrote to a radio host with a lesson about his use of the phrase “my fiancé and I’s…”

Instead I stopped listening. 😀

Especially if you are a non-native speaker like me.

For example, I could rephrase it:
"You are a non-native speaker as I" which is also correct.

In your example it has to be me (my) because it's a possessive So, you would say
It is John's and my car.

I still struggle with all these rules. Fortunately, my wife is a native speaker and excellent writer. I often will ask her to proofread articles before I submit them (I am a scientist, so I regularly have to write journal articles) for review.

But I really get tired of people saying. It was for my wife and I, and the like

Argggggggghhhhhh!!!!!

But then, of all the things to allow to make me nuts, right? I am aware of the silliness of my strong feelings about grammar.

But I still have them!!!

Lol

I heard Rachel Maddow misuse it recently. Wish I would remember that sentence.

"You guys's car" . . . that drives me nuts.

We really need a Nosotros version of "you".
On edit - Laz reminded me that in the South, it's "y'all's car".

Haele

I know an English teacher who doesn't even know when to use me or I and I've corrected her multiple times. She seems to think it doesn't matter and laughs it off. TV shows, movies, and books have characters who say I when they should use me so often that I've wondered how long it will be before the correct way becomes officially wrong.

...just sets my teeth on edge. My own younger son used it the other day, with other people around, and I had to bite my tongue not to correct him. I'm still trying to decide whether or not to bring it up privately. He knows I'm a language nut, so he'd probably just roll his eyes at me.

I find the I/me thing mysterious. It really shouldn't be hard, and no one ever does it wrong in the singular. I have never heard anyone say "Us are going to the store," or "Her had to go to the hospital," or "Give it to I."

And the guys's (guys'es?) things is awful. Rachel Maddow says it frequently and it sounds so wrong coming from someone as searingly intelligent as she is.

on the registration before I could determine the appropriate grammar.

"when I grow up I want to be either a brain surgeon or a short order cook". Jethro

Goes back a long time. I've often thought it's because people think of "me" as ignorant -- as in "me Tarzan, you Jane." Books and films frequently had people who were uneducated in English use "me" instead of "I," and thus "I" became the go-to for all circumstances.

Of course, I could also go on a long rant about how I see this as symptomatic of our society's increased confusion of subject and object, but that might be more than you bargained for.

-- Mal

Using I’s?

What’s next? Me’s and my husband? His and my’s house? 😎

...any ideas.

I can, unfortunately, totally see the "his and my's" happening.

I think I must have been an English teacher in a previous life. Sloppy speech and writing is like fingernails on a chalkboard for me. Years ago, I had a very dear neighbor who always said, "I should have came," or, "I should have went." I really had to fight the urge to correct her.