Your goal, as a storyteller, is to weave a tale so compelling, create characters so real, set a stage so believable, that your reader is pulled into the story and held there, enthralled, right till the end. The ability to hold a reader defines a story's success. You don't want anything to distract them from the world you've pulled them into. Certainly nothing so mundane as poor grammar.
I'm a proofreader and freelance copyeditor (when I'm not doing my favorite thing -- writing). Given my nit-picky nature, I may notice a few more grammatical errors than your average reader, but there are a few gaffes I see over and over again. Here are five common mistakes, and simple tricks to help you avoid them, at least until you're ready to pause and leaf through your dictionary or style book.
Its vs. It's Yes, the hat is Barry's and the shoes are Lucas' and those five bicycles are those five boys'. But when it comes to it, "its" is as possessive as that word gets. There are only three versions of "it": we call a thing "it". "Its" means an object belongs to that it, just as the hat belonged to Barry. "It's" means "it is" or "it has." Keep it simple: "its" is possessive. "It's" is it is or it has.
Affect vs. effect. Affect is a verb, and effect is usually a noun. When you perform an action on something, you are affecting it. The result is the effect. Easiest way to remember? Action and affect both start with "A".
I vs. me. Is it "Harry and I will go," or "Harry and me will go"? Should it be "Give it to Sarah and me" or "Give it to Sarah and I"? Separate the sentence. In the first instance, separate the sentence into "Harry will go" and into "I will go." Both of these make sense, whereas "Harry will go" and then "me will go" does not. Therefore, "Harry and I will go" is the correct form. Ditto on example #2: "Give it to Sarah" and then "Give it to me"; "Give it to Sarah" and "Give it to I." In this case, "Give it to Sarah and me" is correct.
Commas and Cumulative Adjectives My coworkers jokingly call me The Commaphobe because I search and destroy wayward commas. The ones I pounce on most frequently are those separating words that make up cumulative adjectives. Here's a sentence containing a cumulative adjective: Two small black shapes moved toward them. Now, forget the "moved toward them" part of the sentence, and concentrate on the noun (shapes) and on the three words describing the noun (two small black). This is a cumulative adjective, which means you don't slap a comma between each element. At least, not without driving commaphobes nuts.
Here's how you tell if your adjective groups need commas or not: try inserting "and" between each element. "Two and small and black shapes" -- no way! If you can't insert "and", you shouldn't insert a comma, either. Another test: try jumbling the elements around: "Small two black shapes" doesn't work. However, a phrase like "he's a warm, gentle, loving man" passes both the "and" test -- "a warm and gentle and loving man" -- and the jumble test -- "a loving, warm, gentle man" -- so it gets commas.
That vs. which. My personal bane, because the choice has much to do with context. Here's where commas can be your friends, because they offer a clue as to which word should be used. Commas usually signal nonrestrictive elements, or elements that aren't essential to the meaning of the noun -- elements that use "which." Think of "that" as necessary, or definite, and "which" as an added detail. Therefore, a sentence like "The river that runs through our town is wide" is referring to a specific river, whereas "The river, which runs through our town, is wide" is saying the river is wide, and it happens to run through our town. Think: which one? That one.
If you find grammar boggling, don't worry; you're not alone. Nit-picky soul that I am, I've spotted these errors on billboards, movie trailers and in magazine ads. Don't let that lull you into complacency, however. Remember, you've got to hold your readers through an entire story, not just distract them for thirty seconds.