Here are some common words that confuse everyone, not just those of us who want to write professionally (and we should know better):
AFFECT / EFFECT
The verb affect means to influence or have an effect on. Effect as a verb means to bring about or cause; as a noun, it means result. (His opposition affected the outcome of the vote. The vote effected a change in policy. The effect of the change was staggering.)
FEWER / LESS
Use fewer when referring to units that can be counted. Use less when they can be measured but are usually considered in bulk. (Cottage cheese is less fattening than ice cream because it contains fewer calories and less sugar and fat.)
ITS / IT'S
Its is the possessive, just as his, hers, and theirs are. (The cat licked its paw.) It's is a contraction (like can't and won't), short for "it is". (It's a lovely day tomorrow.) Remember one and you'll remember the other. Don't be confused by the rule that tells you to form the possessive by adding 's. Its is the exception that proves the rule.
LAY / LIE
Lay means to place, put down, or deposit. As a transitive verb, it requires a direct object (the thing you lay down). The past tense of "lay" is laid. (Lay the book on the table. He laid it down. She has laid her books next to it. They have been laying things down all over the house.)
Lie means to be in a reclining position or to be situated. It is an intransitive verb and does not take an object. The past of lie is lay. (Let it lie. He lay there without moving. She had just lain down beside him. They have been lying there for hours.)
Lie, of course, also means to tell a falsehood, in which case it is conjugated differently: I lie; he lied; we have lied like troopers; they have lied to us all along.
THAT / WHO
That refers to things (The apple that I gave the teacher had a worm in it); who to people (The teacher who screeched is the one who bit into the apple). If this were not so, Who's Who would be called That's That.
WHO'S / WHOSE
Who's is the contraction of "who is". (Who's there?) Whose is the possessive form of who. (Whose is this blood-stained handkerchief?)
YOUR / YOU'RE
Your is the possessive form of you. (Did you find your keys?) You're is the contraction of "you are". (While you're up, get me an Oreo.) This also applies to their and they're, which is additionally complicated by there, which is neither possessive nor a contraction, so don't use it for either.
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