The number one thing I hate about grammar is when some people (many fondly call them Grammar Nazis) feel a need to correct it in casual conversation. Trust me: I know the difference between their, there, and they’re. I write for multiple publications and got a perfect score on the grammar sections of both the SAT and the ACT (haha yes that’s me being a bit cocky, sorry). So please do not correct me when, in a haste to text you something under a desk quickly while doing ten other things, I used the wrong “there”. It’s because I don’t really care, not because I don’t know.
It takes a certain type of snob to interrupt a conversation to point out that you misspelled a word or said who instead of whom. Unless you are trying to write a professional email or a paper or something else that matters, I promise, no one wants to hear you point out these mistakes. It is condescending, unnecessary, and just plain annoying. Those who want advice will ask for it.
On the other hand, to genuinely not have these rules mastered do indeed look foolish, especially in a community of educated people. Because if you take the 5 extra seconds to think about the rule when you are trying to write properly, it’s really not that hard. As something I once saw on Tumblr says, “Grammar: The difference between knowing your shit and knowing you’re shit”.