So, let's talk grammar.
The good news: The rules of grammar can be bent, twisted, and in some cases, broken, in creative writing. As opposed to formal writing, in creative writing, we're allowed dashes (which I adore!), fragments (which I also adore), run-ons (which I personally don't adore), and many other errors that a formal writing teacher would be happy to slay with one stroke of her bright-red pen.
The bad news: The rules of grammar cannot be totally thrown out the window in the name of creative writing. Things like too many run-ons, lack of proper format and punctuation, misspelled words (accidental), wordy/wandering/meandering sentences that have no clear focus, etc. These should not be acceptable, even in creative writing.
Which brings me to this: if a writer cannot master the elementary basics of grammar, the reader will learn quickly not to trust him/her. And, worst of all, poor grammar distracts the reader from the point being made.
I've seen this happen over and over again with my students - they'll have a nail-biting plot, smooth dialogue, deep characterization - but I'm too busy marking the myriad of run-ons, the lack of punctuation surrounding dialogue, the missing indents for each new line of dialogue, and the misspelled words, to see the good stuff. Sure, as a teacher, I'm actually looking for that sort of thing. It's my job. But - the reader will catch it, too, and will be distracted from the sparkling text.
Bad grammar always gets in the way. It's almost like a crimson wine stain on an otherwise-pristine white carpet. What's our eye drawn to? Always the stain.
And, honestly, agents/editors won't give a stained-with-bad-grammar manuscript a second glance. They'll look at a poorly-written page, no matter how brilliant its text, and move along. On to the next writer.
So - know the basic rules of grammar first. Honor them. Only then are you free to bend them.
In the words of T.S. Eliot: "It's not wise to violate the rules until you know how to observe them."