Here's an alphabetical list of frequent usage errors that annoy instructors and many readers. Getting them right is an easy way to make a positive impression on the reader.

• a lot is not spelled alot. These are two words, just like a few.
  
  
• all right is also two words.
  
  
• amount, number; less, fewer: Do not use the words amount and less when the quantity involved can easily be counted (except in the case of money). When you are in doubt, let the words much and many be your guides. If much will fit, the words amount and less are correct. Many corresponds to number and fewer.
  
  WRONG: The Giants score less runs than anyone. (You wouldn't say "much runs.")
  RIGHT: The Giants score fewer runs than anyone.
  
  WRONG: She knows a large amount of people. ("You wouldn't say much people.")
  RIGHT: She knows a large number of people.
  
  
• between you and me
  WRONG: between you and I. Why? The pronoun is the object of the preposition between, so you should use the object case, me, rather than the subject case, I.
  
  
• is when, is where. Do not use these expressions in definitions. The result is an error in logical equivalence.
  
  WRONG: A monarchy is when a country is ruled by a queen or king. (A monarchy is not a point in time.)
  RIGHT: A monarchy is a country ruled by a queen or king.
  
  WRONG: A newspaper is where you get the latest misinformation. (A newspaper is not a location)
  RIGHT: A newspaper is a publication that prints the latest misinformation.
  
  
• it's, its: It's with the apostrophe is a contraction for it is: It's time to leave.
  Its without the apostrophe is a possessive, like hers and his: The ship lost its mast.
  
  
• should have, could have, would have: The second word in each case is have, not of, even if it sometimes sounds like of when people say it.
  I should have seen to this matter sooner.
  He could have come over if he had wanted to.
  I would have sent you a card if I had had your address.
  
  
• their, there, they're:
  Their is a possessive: Their taste was questionable.
  There is a word that indicates location or existence: There is the old factory. There are two kinds of table salt.
  They're is a contraction for they are: They're good friends.
  
  
• to, too: To is used as part of a verb form called an infinitive or as a preposition:
  To run in the park after dark is dangerous.
  He went to the store.
  Too is an adverb meaning also or more than enough.
  I went, too.
  You're too generous.
  
  
• who./whom
  
  
• Use what I call the "m & m" guideline to figure out which one to use: would you use they or them in that same space? I know the man {who or whom?} you were referring to. Would you say you were referring to they or you were referring to them? The one with the m ending, them, right? So use the m ending one: I know the man whom you were referring to.
  
  
• whose, who's: Whose is a possessive: Whose key is stuck in the lock? Who's is a contraction: Who's hungry besides me?
  
  
• your, you're: Your is a possessive: Your car is missing.
  You're is a contraction for you are: You're leaving too early.
  
  
• used to, supposed to: Don't leave off the d's when necessary, even though
  you may not hear them when people speak.
  I used to play cards regularly.
  The guardian is supposed to be an adult.