I've written a novel and looking back the story line was good but the grammar wasn't great. I submitted the book to a few agents and received some constructive comments which I took on board. I have rewritten the story and corrected many of the errors. For instance I removed gerunds. Now that it's complete I'm getting hung up on other issues for instance starting sentences with quickly. I don't want to put agents off accepting my work but does this honestly matter? Should I just bite the bullet and start submitting the novel again?
When you submit to an agent or publisher, your writing is all they have to go on when forming an opinion of your work. You want to make the best impression that you can.
Think of it this way: If they're considering two equally engaging stories, where one will need significant line-editing to fix the grammar, and the other doesn't, they'll want to buy the story that requires less work to publish. I think it will be worth your time to give it one more pass to tighten things up.
Your grammar doesn't need to be perfect in fiction and breaking certain rules is perfectly acceptable so long as you convey the correct meaning. An agent isn't going to turn down a brilliant story because it needs to be proofread.
However, if you know full well that there are lots of grammatical errors and it will require significant re-writing at some point, then clearly you do need to fix it before you send off your manuscript. Especially for the beginning of the manuscript.
If you've noticed the mistakes that implies you are capable of fixing them. If you can't be bothered to present the best work you can -- why should anyone else bother with it or take it seriously?
While I wouldn't consider "gerunds" (or even adverbs) to be mistakes, if you're worried about your grammar, hire an editor to do a line-edit. Explain (if this is the case) that you're happy with the story and don't want a content edit, but you do want to polish your grammar, structure, word choice, spelling, and so on.
Publishers do have editors, but if your work is not already the best it can be, why would they waste their time and money on you?
So yes, IMHO, it does honestly matter, and you should make the effort to clean up your manuscript as much as humanly possible.
When you send your work to a publisher or agent or anyone, it should be the best that you can do. Which would you rather read: A story that is the writer's absolute best effort, or a story that he figured was "good enough" and he was too bored with to bother working on any further?
Many writers crank out a first draft hurriedly as thoughts come to them, and that draft includes many grammar errors, clumsy wordings, incomplete thoughts, etc. That's fine ... as long as you go back and clean it up later.
Why would you NOT fix the grammar errors? Is it because it's too much work? If that's your answer, then I can only say, knuckle down and do it. Producing a quality product -- whether a novel or furniture or a gourmet meal or whatever -- requires having the discipline to do what it takes to get it done right.
Is it because you're anxious to see your work in print? But if grammar errors result in your book being rejected by agents and publishers, then in the end you may well have to fix the errors and start all over, and it will only take longer. And if you made a bad impression on your first submission, they may not be willing to seriously consider your second try. When my books got close to being ready to go to print, I rushed some things because, hey, I've done all this work and now I want to see it in print. But that's a bad attitude. Don't throw away months of work because you got impatient at the end.
Is it because you don't know grammar rules well enough? Then you need to take some time and learn them. Get a good book on grammar. If I buy a car and it doesn't run, and I take it back to the dealer and he tells me, "Well, yes, but the engineer at the car company doesn't really know a lot about auto mechanics so this was the best he could do", my response will be that I want my money back. Publishers and readers want a good, quality book, not excuses about why you couldn't produce one.
Is it because you don't think anyone cares? They do. Grammar errors make a book hard to read and understand. They make the author look unskilled. This is a novel, but imagine you were writing a non-fiction book and found it was filled with spelling errors. At some point you would say, If the author doesn't even know how to spell (say) the word "astronomy", why would I think he knows anything about the subject? Yes, we all make mistakes. If you have two grammar errors in a 300 page book, likely no one will think much of it. But if every other sentence has errors, readers are going to give up on you.
Agents and publishers get so many manuscripts that they have to screen them quickly. Anything, especially in those first few pages, that makes you seem no better than average will get you rejected.
Most published books are commercial failures. I read that 90% of books for which royalties were advanced do not earn out the advance. Since publishers know they are shooting craps when they select a book, they are as fussy as they know to be.
Big publishers don't do the degree of editing that they used to, in order to keep labor costs down. So a first-time author needs to have their work professionally (or at least competently) edited before submission.
Get yourself a copy of the "Chicago Manual of Style" and read relevant parts.
I suspect, too, that if your grammar is shaky, there are other things wrong with your writing. Although it is targetted for mystery writers, I strongly recommend fiction authors of any genre read "Don't Murder Your Mystery" by Chris Reorden. It's not really about writing mysteries, it's about writing fiction well, heads above your competitors--who are also sending their manuscripts to the same people you are.