My Grammar wanted us to learn good English!

It's "it's" and "its" for me. My RoT is make the apostrophe into an "i", and see if the sentence still works.
The "horriblest" abuse I've seen was a British author writing about a young man who was an English Major in college, but did "U-Chooz". All the "its" and "it's" were "its' " 😲 ("itses"? Sounds like Gollum, yes it does, my precious!)

And (at no extra charge!) the article missed "yore"! "Your father said, 'Pretend you're in days of yore.'")

My dear brother who loves to share this topic with me made an error that I didn't point out. Once I argued with my husband that Lloyd has only one L. As if it could have a silent L....ya gotta be kidding!