1. It’s vs. Its
This is a common mistake. It’s also easily avoided by thinking through what you’re trying to say.
“It’s” is a contraction of “it is” or “it has.” “Its” is a possessive pronoun, (easy to mistake, as most possessives DO get apostrophes -- just not this one. Here’s an easy rule of thumb—repeat your sentence out loud saying “it is” instead. If that sounds goofy, “its” is likely the correct choice.
2. Your vs. You’re
This one drives me nuts, and it’s become extremely common, even among writers with good skills othewise.
“Your” is a possessive pronoun, as in “your car," "your computer” or “your book.” “You’re” is a contraction for “you are,” as in “you’re screwing up your writing by using your when you really mean you are.”
3. There vs. Their
This one seems to trip up everyone occasionally, often as a pure typo. Make sure to watch for it when you proofread.
“There” is used many ways, including as a reference to a place (“let’s go there”) or as a pronoun (“there is no hope”). “Their” is a plural possessive pronoun used when something belongs to more than one someon, as in “their bags” or “their opinions.” Always do the “that’s ours!” test—are you talking about more than one person and something that they possess? If so, “their” will get you there.
4. Affect vs. Effect
To this day I have to pause and mentally sort this one out in order to get it right. As with any of the other common mistakes people make when writing, it’s taking that moment to get it right that makes the difference.
“Affect” is a verb, as in “Your ability to communicate clearly will affect your income immensely.” “Effect” is a noun, as in “The effect of a parent’s low income on a child’s future is well documented.” By thinking in terms of “the cause producing the effect,” you can usually sort out which is which, because you can’t stick a “the” in front of a verb. While some people do use “effect” as a verb (“a strategy to effect a settlement”), they are usually lawyers, and you should therefore ignore them if you want to write like a human.
5. The Dangling Participle
Now I can dangle a participle with the best of them and once had a woman lying in a barn with a broken hip. My dear hubby (who was proofing for me) saw that, laughed and said, "Must've been the roof!"