Presenting as clean a copy as you can to an editor makes a good impression on your level of professionalism. Using the correct forms of the verbs, properly distinguishing between "to", "two", and "too" is important as is figuring out where to put the commas. Spelling, especially in this day of spell-checking software, shouldn't even be an issue.
That said, I remember the first time, many years ago, when I read the first-draft manuscript of a book that had just sold to a major publisher. I was appalled at the mechanical errors. When I (gently) pointed one out, the author flipped her hand at me and said, "That's what editors are for."
Since the mss had sold, I had to believe she was right.
Yet my spirit rebelled. How could someone who took writing seriously as a profession not care about the tools we use?
I've since spoken with many editors and, while there are some who are focused more on story and are willing to ignore bad mechanics, many of them were far more willing to roll their eyes and complain about them. In fact, more than one told me poor spelling and bad grammar were automatic rejects, in their opinion. That, if an author couldn't bother to take the time to learn how to purposefully use the language, then he/she as an editor didn't want to take the time to read the manuscript.
The key in there is "purposely use the language". Yes, characters exist that don't use proper English (Huckleberry Finn, anyone?). But Mark Twain made choices about his word usage. He didn't write out of ignorance.
Today, we're going to take a look at your choices.
Open your current work in progress or a finished work that isn't yet published.
First step is to use the editor that's in your wordprocessor program. I'm a fan of Microsoft Word 2003 (sidenote: I HATE the ribbons that have "improved" the newer versions. Microsoft dropped the ball on this one. They took something that wasn't broke and tried to fix it!).
Okay, off soapbox. I like Word 2003 because I can put all my tools on the top of the page and I can set the style editor to check my grammar and style as I go (Tools - Options - you can set most everything in the menu box that pops up). While I've always been conscious of my grammar, I'm not a style maven, so I'm grateful for the little green and red lines it puts under my words as I write.
When I'm ready for a break from the creative side of writing, I go back and made conscious determinations about each of those lines. Remember, everything you write should be purposeful. If something doesn't fit with what Word wants, it's because I want it that way.
Second step: Word doesn't catch everything. It won't look for comma splices or dangled modifiers. You need a different program for that.
I use AutoCrit, an online program that's VERY thorough. This is a site I pay for although the free tools are pretty good, especially if you're just starting out. I like the extra tools I get with the subscription (and the longer length to the manuscript checked). It will help me find not only my overused words but will helpfully point out cliches, frequently used phrases -- all sorts of style problems.
AutoCrit is good because it makes no changes. It simply points out and I make the decisions about what stays and what goes. Purposeful writing!
Wordle is wonderful for those of us who are visually stimulated. It takes our most-often used words and makes a collage out of them, presenting our manuscript as an artwork.
This can help a writer quickly see the overused words (when "back" is the largest word on the page, you know you have a problem!). You can also right-click on words to remove them from the image. Since character names are usually my largest words, I remove those so I can see what's left.
Once I have my Wordle, I find a black & white, easily read version (use the "random" button at the bottom to change the image) and print it out. It goes on my wall next to me and I use Word's "find" function to highlight and edit my overused words.
Grammar is important. It's one of your tools. Use Word. Use AutoCrit and Wordle and you'll find, after a while, you begin to learn grammar you never knew.
Have fun and leave a tip in the jar :)
PS. Here's a guy who won't even look at hiring you if you have poor grammar. Good article!