As I attempt to write a basic creepypasta, I periodically copy the newly written paragraph and paste it into a grammar checker. I used to use Scribens which didn't point out unseen errors as well as I thought it did. Then I tried to use Grammarly, but I can't sign in without a Facebook or Google account (to which I can not make a Facebook account without the permission of my parents) and I am struggling to log in to Google. For now, I will have to result in using a different grammar check software, but I can't seem to find a powerful one.
Heya, DrBob has created a list of alternate grammar checkers. However, if you want to use Grammarly, you can sign up here without a Google or Facebook account (you just need an email).
Each tool has advantages and disadvantages. I use a minimum of three (Scribens, Grammarly and ProWritingAid) before even putting a draft up on the Writer's Workshop.
Grammarly does a good rough pass, especially if you put the time into it to deduce all those "Premium" errors. It isn't perfect. You can pass Grammarly and still have nonsense.
Scribens is the weakest of the three, but does some things really well for sentence analysis like run-ons and unneeded words. Forget its sentence formatting errors. REALLY bad - ignore them.
ProWritingAid does a great job, on 500 words or less. I write in Word and put in about one page at a time.
None of these is 100%. Sometimes, you have to say "That's not how real people talk" or "That's a bug." That said, more than 95% of the time when Grammarly or ProWritingAid says it is a problem it really is.