@ThanksItHasPockets
As explained now by multiple posters, English is relatively easy to learn for the purpose of functional communication but difficult to master to fluency.
Exactly this. I know quite a few people who came here from other countries, in their 20s or 30s (some 15-25 years ago,) and they can communicate in English/speak English OK. But it is quite basic with most of them. Even after 15 to 25 years of living here.
I'm not knocking them. I think it's absolutely brilliant that they can speak English - and I'm really impressed by them learning to communicate in another language.. But at the same time, as I say, it is basic, limited English because it IS a really hard language to learn properly. Even people who were born here/raised here struggle with it for goodness sake!
I don't think there's actually anything terribly wrong with correcting people sometimes, because people need to know they're doing it wrong, so at least they can try and correct it if they can, (although it doesn't matter if they do get it wrong as no ones perfect!) 
But it's the patronising and demeaning and obnoxious way that some people go on about it, (especially on message forms like this,) and the way they put people down, and mock, and berate, and deride them for getting it 'wrong...'
Like, you know, 'you must be a bit thick' putting TOO instead of TO, and 'could of' instead of 'could have' ... It's just the snarky 'I'm much better than you because my English is better than yours' attitude.
My husband's English and grammar is a little bit questionable, even though he's an intelligent man and very good at things like I. T. electrics, carpentry, and art... Even he thinks that he's a bit 'thick' (and not as intelligent as me,) because his grammar and spelling is not brilliant and mine is quite good. He's no more 'thick' because his grammar and spelling isn't brilliant - than I am 'super intelligent' because mine is reasonably good. I am also not in a higher social class than him because my spelling and grammar is a bit better, (like someone suggested further back in the thread!) We are both working class!
I am fairly good at English, spelling, and grammar, but do make mistakes sometimes, and I am definitely no more 'intelligent' than anyone who isn't very good when it comes to the English language!
The snobbery (from some) around spelling and grammar is horrific. I'm embarrassed for them. Also, as a number of posters have said, language evolves, and some people are not 'wrong' in what they are saying. Also, what some people claim is something being said 'incorrectly' is just someone's regional dialect. Drop the snobbery! It's not a good look!