[quote Geamhradh]@SchrodingersImmigrant, I think, as in all professions, the fact that you're good at one thing, doesn't necessarily mean you're good at others. Rightly or wrongly.
It's also true, and a sad indictment of the school system at the time, that when I was at secondary school (1977-84) there was no National Curriculum and schools did whatever they wanted (basically) Mine did no English language whatsoever. We did a subject called Literature and Drama. A 2.5 hour slot a week where sometimes we read extracts from anthologies, and sometimes we pretended to be seeds. 
I did Modern Languages and Linguistics- hence phonology nerdiness and I'm an English teacher now. Everything I know about English grammar, I learned from linguistics, or from extrapolating rules from other languages, or when I had to actually start teaching English. Factor in that a lot of people choose not to take a MFL once it becomes optional, so they're not given the extrapolation option, and you've got a lot of teachers (as in any other profession- I was at school with someone who now works at NASA, he did A' level Physics at 13, couldn't get his O'level English for love nor money. He was a year older than me but went to uni a year later because of English) whose use of English is substandard. Arguably, if they're not English teachers, does it matter? I don't know.
I firmly believe though that back in those days, more than one generation was failed by the system. And some of those people, like me, are teachers now. I had to collate student reports a few years ago, and one NQT's had to be redone because she'd written "could of" and "should of" throughout.
I'm a descriptivist not a prescriptivist (or worse, proscriptivist) though, and so the threads I'll die on are the ones started to belittle others for their less than perfect use of English. Because yes, the answer very often is "no, they don't read", "no, they don't know it's wrong"[/quote]
I've seen "vars" (Vase) and "payed" (paid) today alone...
I remember studying Dutch grammar, and correct grammar really is pushed from the very beginning. You learn all the rules, sub-rules and exceptions, and the indefinite article has two forms. I don't ever remember being taught English grammar. That said, its really not very difficult to pick up in your own language, but there seems to be almost a reverse snobbery thing going on about correcting people and using correct spelling and grammar.
What I really don't understand is how people struggle with plurals in English. They are really easy - you mostly just add an "s" or "Ies" onto the end, but people put in apostrophes or don't know when to use "ies" as if they have never, ever been instructed how to create a written plural in their own native language.