(This was actually an article about resume writing in Vault magazine) but it holds true for our daily writing as mystery shoppers:
1. Accept/Except
To accept is to receive something. She accepted his apology. Except is a preposition that means but or with the exception of. I would use accept, except it is not the correct word.
2. Affect/Effect
To "affect" is to influence or change. He affected her emotions. An "effect" is the result of something, as in cause and effect. Typically, affect is a verb, and effect is noun.
3. And/Or
This usage implies three outcomes, not two. I will write a resume and/or a cover letter means that the person will either: (1) write a resume; (2) write a resume and a cover letter; or (3) write a cover letter.
4. Assure/Ensure/Insure
To "assure" is to convince or to guarantee. The administrator assured him that his resume had been received. "Insure" means to guard against loss. I insured my car. "Ensure" means to make certain. I ensured that I insured my car.
5. It's/Its
This is one of those cases that is an exception to the rule. While the apostrophe typically denotes a possessive, in this case, "it's" means it is, and "its" is the possessive.
6. Their/There/They're
"Their" is a possessive; something belongs to them and it is theirs. "There" is where something is; it is over there. "They're" is where they are; they're (they are) over there.
7. Then/Than
"Then" is when something will happen and means next or consequently. I will go to the store and then go home. "Than" indicates a difference. Chocolate is better than vanilla.
8. To/Too/Two
"To" is a function word indicating an action or process. I want to write my resume. "Too" indicates an excessive amount of something. I am too tired to write my resume. "Two" is the number that follows one and precedes three.
9. Utilize
"Utilized" means use. Use "use" and not utilize.
10. Whose/Who's
"Whose" is a possessive. Whose resume is it? "Who's" means who is. Who's at the door?
11. Your/You're
"Your" is a possessive. It is your turn. "You're" means you are. You’re correct.
For more commonly misused words, there's a great list over at Oxford Dictionaries. Another option is to conduct an online search for "commonly confused words" and you'll find plenty of results to choose from.
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