As a native English speaker who studied writing at university, do you think "who" should be used with people while "that" should only be used with things or the other way round. Or should I just not care?
Edit: missing things
Use 'who' with people especially, often with other living beings ('my dog, who runs away daily, always is home for dinner') or groups of them ('the NY Yankees, who won the championship that year, were my favorite'), but never with objects unless pretending they live ('my stuffed bear, who sleeps in my bed, wakes me every morning').
If you care about these things, the Chicago Manual of Style is a large, technical, highly respected guide aimed at publishing. Fowler's Modern English Usage is more focused on usage. A short and beloved guide is The Elements of Style by Strunk & White. You can find all on the Internet Archive, I'm almost certain.
It gives clear, practical advice in a very accessible style and format. If you have any comparable substitutes, I'm all ears.
Well played.
This, unless you're being tested on it. Maybe the safest bet is to avoid it. "As a native English speaker that studied..." oh wait shit lol, it's actually quite hard to avoid.
"I'm a native English speaker, and I studied writing in university. This experience has led me to..." There. English, what an uncomplicated, uncluttered language!