Common Grammar Mistakes

grammar mistakes

It’s so easy to make mistakes when writing. Without the threat of a pop quiz from your English teacher, the rules of grammar tend to fade away. That’s fine in everyday conversation and personal emails, but professional correspondence has stricter requirements. A mistake in spelling, punctuation or grammar can reflect badly on you, indicating sloppiness and inattention to any reader who catches the error. No pop quiz, but still, the threat of failing is there.

Avoid that possibility by learning the rules that you’ll use most often, and looking up correct usage when you have doubts. Following are some of the most common mistakes people make when writing:


Use “they’re” whenever you mean “they are,” and “their” when you are indicating ownership. Any other time, you are probably looking for “there.” E.g. “I told the Smiths they’re always welcome, as long as they don’t bring their pit bull.”

Could of/should of:

Yes, that’s how it sounds in conversation, but the correct phrase is “should have” or “could have.”


Just remember that a person is always a who, not a that. “The woman that gave me the medal” should instead read, “The woman who gave me the medal …”

CDs, Hondas:

Don’t use an apostrophe to indicate that a word is plural. If you have more than one CD, then you have CDs, not CD’s.

Over/more than:

When talking about amounts, use the phrase “more than” rather than “over.” E.g. “The album sold more than 3,000 copies.”


Use the contraction “it’s” when you mean “it is,” and use “its” to indicate a possessive. E.g. “It’s clear to me that a leopard can’t change its spots.”


Similar to the previous rule, use “who’s” when the meaning is “who is,” and “whose” when the meaning is possessive. E.g. “I’d like to know who’s responsible for determining whose domain the project falls under.”


To “affect” something means to influence it. Effect, as a noun, means “the result.” Chances are if you’re looking for a verb for your sentence, the correct choice is “affect,” and if you need a noun, you probably need “effect.” E.g. “I hope the rain won’t affect your plans to visit, but I know lousy weather always has a detrimental effect on my mood.”


When the phrase that comes after the word is an essential part of the sentence, you should use “that.” When the phrase is not essential to the sentence, just an added detail, use “which.” E.g. “The shoes that caught my eye yesterday are too expensive. But I found a similar pair, which also come in black, marked down to $30.”

Quotation marks:

Quotes are primarily used to indicate what someone said. They should only rarely be used for other purposes. Overusing quotation marks for emphasis is a common mistake. When in doubt, leave them out. And always place your punctuation inside the quotes. E.g. “I was devastated when James Brown died,” said Nancy. “He was the Godfather of Soul!”


When the words inside parentheses form a complete sentence, place the punctuation inside. When they don’t form a complete sentence, the punctuation goes outside. E.g. “For Christmas, my boyfriend gave me a dog. (What a sweetheart!) I named him Orson Welles (after the famous director).”

Like/such as:

Use “such as” when you are giving an example, and “like” when you are indicating that something is similar. E.g. “My dad says octopus tastes just like chicken. But I prefer less exotic meals, such as pizza or pasta.”


You pay someone a “compliment” when you say they’re looking nice today. Things that “complement” one another go well together. E.g. “The merlot that John brought really complemented the flavors of the meal.”


The first issue of your publication is its “premiere” issue. You might hope that it could also be described as “premier,” or the best in its class.