Easy mistakes to make, but fortunately, easy ones to correct. Scan this list to make sure that you are not "grammatically incorrect"!
Eight MISTAKES (TO AVOID)
1. "Its" and "it's." Quick quiz: do you know which of these words is the possessive form of the pronoun "it"? Correct answer: "its". I have seen the incorrect "it's" on signs, ads, and other places. It screams out that someone is careless. "It's" is the contraction of "it is". But you knew that.
2. "There" and "their." A variation on #1, but also prevalent. "Their" is the possessive form of they. Oftentimes, people are so used to writing the word "there", they do it automatically. Of course, because it is still a correctly spelled word, albeit an incorrectly used one, computer spell checkers will not pick up the mistake.
3. "Complimentary" and "complementary." The first word means something that is free ("complimentary parking for customers") or receiving praise. The second word means "serving to fill out or complete". Anyhow, why not just say that the parking is free?
4. "Principal" and "principle." The first was the person in your elementary school that evoked fear in your heart. Building on that, "principal" is used to mean main or leading ("the principal causes were...."). The second refers to ideas, theories, etc. ("the principal stood by her principles and suspended me for talking back.")
5. Nominalizations. Avoid them. Nominalizations are long strings of words, often peculiar jargon to a particular field. Taken together, they can be hard on the eye, and forget ever reading them aloud.
6. Too many qualifiers. True, if you say that "all children love ice cream," you will hear from someone that his six-year-old son can't abide the stuff. Still, when we over-qualify, our writing sounds tentative and stilted. Write it out, qualifiers and all, then go back and see if you can cross out some of the extra verbiage and still retain the meaning of your sentence.
7. "That" and "which." Use "which" when the information that follows is optional to the meaning of the rest of the sentence. For example, "he wore his raincoat, which his aunt gave him last year." In this case, he has one raincoat, and we're just learning more about it. "He wore the raincoat that his aunt gave him" implies that he has a few raincoats, and we are learning which among them he chose to wear.
8. They/he/she. "The visitor will check in with their credit card." Of course, we don't want to say "he" anymore unless we know are referring to a specific male person. Two fixes: "visitors will check in with with credit cards" (preferred). Or "the visitor will check in with his or her [or "her or his" for variety] credit card."