There’s no getting around grammar. Proper grammar is needed for sentences to convey the information you need to. Without knowing it, the reader needs clear definition of subject and object in every sentence.
Mike dropped the ball.
Mike is the subject performing the action. The ball is the object of Mike dropping.
Mike and the ball both being nouns (person place or thing).
“Dropped” being a verb.
You need THAT MUCH grammar to write a book. Preferably more.
How much grammar do you need to know?
Let me get this clear – you don’t need to be an expert on the Oxford Comma.
But it can’t hurt.
The more you know about language, the better you will use it.
Should you get a degree in English grammar?
Again, not necessary. Some college courses couldn’t hurt – but you can audit those through MIT open courseware, or any number of English Grammar courses.
It helps also to try a Pimsleur language course. You’ll see that the grammar of other languages differs greatly from English (for instance, English lacks the masculine-feminine tenses other languages have). Learning a few Pimsleur courses of another language will improve your grasp on English grammar a great deal.
Regional and ancestral differences
My English reflects (apparently) a midwestern influence (thanks to my mother) and a slight southern influence (thanks to my father), a strong New England influence (thanks to my youth), and sometimes the word order I use is confusing to English readers who’ve never encountered a Yiddish influence (thanks to my ancestry).
In Yiddish, modifiers can be placed at the end of the sentence,apparently.
Knowing this has helped me to root it out and kill it when editing my writings – except when I’m writing dialog for Jewish characters.
I’m very good at writing dialog that (if you know what you’re looking for) instantly identifies the speaker as German, Jewish, Scottish or British. I also do a passable Japanese, thanks to many years of Martial Arts. Oh yeah, learning traditional martial arts forces you to learn a lot of Japanese.
Know and identify regional differences in language. Know and identify ancestral differences. Knowing how to identify them helps you to know when to use them and when to eliminate them.