English is not an easy subject to learn, so many difficult spellings and sounds (especially if spelled correctly not the US way!)
There has been much change in the meaning of words.
I use to compete on a horse called Gay Boy. I thought that was a lovely name - which it was in the original meaning of gay.
I caused quite a lot of consternation and curiosity when in the US and asked a butcher for a joint (roast) and remarked when in a restaurant, "I'm going out for a fag!" (Cigarette)
I try to not correct youngsters I am around all the time. It can alienate them but it does grate when you keep getting "I were going." or "We was going."
Grammar does matter. I would always employ someone who spoke correctly over someone who didn't as long as both were capable.
I do not know if any of you watch Super Nanny but Jo Frost, brilliant with not only children but people too, would not be employed by me because of her bad grammar. (Not that she would ever want a fixed job.)