Quote by Notbob
I haven't read all the responses. Grammarly has (or at least used to have) a free version to download and use. It will help with grammar and punctuation. One tip...don't double hit the "Return" key to separate paragraphs. It creates a space much bigger on the site than it does in Word. It's a mistake I've made, and Lush won't accept the larger spaces. Only hit the Return key once to separate paragraphs.
Grammarly does still have a free version, which I used on my Dick Job story a few months ago after getting rejected twice for dumb errors. Even though I consider myself a good writer and am very familiar with rules of grammar and punctuation, I had a number of silly mistakes that were obvious once pointed out. As has been said above, we often are so familiar with what we just wrote that we gloss over it and don't notice the mistakes.
A tool like Grammarly won't catch all your errors, and it will flag things that are not necessarily errors, depending on the context. In particular, I disagree with it about the Oxford comma, but nevertheless it has been tremendously helpful with my last several stories, and I know when to ignore it. Also, it is intermittently a real resource pig, often slowing down my Mac massively even when I'm not doing any writing. I turn it off 95% of the time, and turn it on only when I'm preparing a story for submission.
Also as others have said, most people can't just write down their stream of consciousness, immediately think it is well written, hit Submit and get it accepted. Writing good stories that are easy for others to follow (and like it or not, if you want people to clearly understand what you are trying to write, you need to follow most of the rules), and hopefully hot enough to get them jerking off to boot, is a lot of work. After getting the initial thoughts and story structure down in a first draft, I revise it AT LEAST five to eight times, going all the way through from start to finish, making significant changes and improvements every time. For Dick Job, I probably went through all 10,000 words at least fifteen times, and each time I found things to improve. That's more reading than the length of a typical novel.
My latest, Pork by Northwest, went more quickly than that, but I think I still crawled through the text eight times or so. This time around, it didn't get reviewed as soon as I'd hoped after I hit Submit. While I was waiting, I reread it, noticed a couple of typos and Withdrew it to fix them. In that process, I discovered 30-40 more other improvements to make to the text before resubmitting. Glad I did.