Grammar, punctuation and spelling are the tools we use to convey subtle meaning to the reader. Even if the reader doesn’t know enough about grammar to truly appreciate how we’ve done it, they will respond to what we’ve done. The word grammar derives, via Old French and Latin, from the Greek grammatike tekhne meaning ‘art of letters’. It is an art – you’ll notice I said fashions of grammar rather than rules. Grammar is what the majority agree is an understandable way to structure language. It is how we order words so that we can communicate.
The ‘rules’ change, sometimes slowly and sometimes quite fast. What you were taught at school may or may not be correct and I guaruntee that, unless you did linguistics at university level, it avoided the glorious complexity of language structure. Grammar in written English is different to grammar in spoken English. Lolspeak becoming a language of its own which is incredibly exciting. The writer who gets what grammar does and how to use it in both expected and unexpected ways has the power to use her words to maximum effect.
An excellent resource for improving your grammar is the Daily Grammar Lessons blog. The other is reading a lot.