This is a major no-no. Why? Because you are falling into one of two pitfalls. Either you're writing the entire book without proper grammar or your dialog is jerky and awkward.
Most authors I read fall into the second category. They apply the same rules to their dialog as the rest of the writing and you end up with dialog that doesn't feel right. It doesn't feel natural. It isn't actually how people speak. Now, I'm not advocating making speech too realistic. As I said in a post back in October, it's painful to read (a portion of the post is quoted below).
The other point has to do with readability and accents or ethnic origins. We all want to be true to the background of our characters. You want that Latino to ring true in the reader's mind. However, you don't want choppy dialog like, "Eeees good ju don go, vato." That wasn't too bad and there are far worse examples (that was hard enough for me to write but, again, I've seen worse). The above should never, never, never, never be done. It's choppy and hard to read. It's painful to read. Instead, if you want the ethnicity to ring true. Pay attention more to how they forms their sentences than how they form their words. Like, for example, in the Spanish language, they rarely ever use pronouns. So saying, "Is good you no go, vato," is okay. That is very close to the Spanish sentence of, "Es bueno no vas, vato," with only the you being adding in English. It's much easier to read and you get across the ethnicity of your character.
But even when not dealing with teenagers, this is important. For example, too many authors don't use contractions (or don't use them consistently) in dialog. Listen to people. How often do you hear someone say "there is" instead of "there's"? What about "do not" vs. "don't"? People use contractions constantly in speech. We rarely use the alternate form unless we're trying to emphasize something, like a parent saying, "There is no way you're going to that party!" So, unless you're purposefully trying to make a character sound stuffy, upper crust, or old fashioned, use contractions.
Remember, your goal is to make your characters ring true for your readers, suspend disbelief. Dialog is a large portion of most fiction writing. Use it.
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