1. That/Who
      refers to things, who refers to people and personified animals.

      He is the one who ate the fish that ate the worm.
    2. Who is the subject, whom is an object.

      How can you tell? Try substituting him or her. If it works, then use whom, otherwise, use who. This one is tricky, so check with a teacher if you're not sure.

      The only person for whom the house was in any way special was Arthur Dent, and that was only because he was the one who happened to live there.
    3. Effect/Affect

      Both words have several different meanings, but the most common are to use affect as a verb, and effect as a noun.

      It says that the effect of a Pan Galactic Gargle Blaster is like having your brains smashed out by a slice of lemon wrapped round a large gold brick. It will affect even a Vogon.
    4. Lay/lie

      This one is really confusing.

        Present Past Helping
        lay laid had laid
        lie lay had lain
      1. In the present tense, lay is what you do to something else, lie is what you do yourself (I lay the fork down on the table, I lie down on the bed).
      2. The past tense of lie is lay (confused yet?) and the past tense of lay is laid.
      3. With a helping verb, lie becomes lain, and lay becomes laid.
      4. And that doesn’t even count other meanings, such as telling a lie (or lying). If you run into this, best ask a teacher for help.
    5. Of/Have

      I should have done it, not I should of done it.
    6. I/Me

      If you’re not sure which one to use, take out all the other names and pronouns, then see what works.
      ex: Should we say, “She gave a cookie to Harry, John, and I?” Take out Harry and John. Would you say, “She gave a cookie to I?” No? Then use me.
    7. Whose/Who’s

      Who’s is a contraction for who is. Whose is the possessive of who.
      ex.: Whose brother is the one who’s going with us?
    8. Kind of/Sort of

      Both mean “a type of.” Neither means “a little bit.” Better to use “rather” or “somewhat.”
    9. Different than/from

      Use different from in most cases. Exception: if it is followed by a subject and verb, use than.
    10. Try and

      Never! Use try to.
    11. Then/Than

      Then is for time, than is for comparing.
    12. Like/Such as

      Like is for comparing, such as is for examples.
    13. Bored of

      Never! You can be bored with something, or bored by it.
    14. On accident

      Never! It’s by accident.
    15. Shined/shone

      Shined is what you did to something, shone is what it did by itself.
      ex.: I shined the penny until it shone like a diamond.
    16. Hanged/hung

      Hanged refers only to killing someone by hanging: otherwise use hung.
    17. Waked/woke

      Actually, either one is fine — just pick one and stick with it.
    18. A/an

      Use an when the next word starts with a vowel sound.
    19. Except/accept

      Except means "but" or "leaving out," as in Everyone except me got to go."
      means "to receive willingly," as in I accept your apology.
    20. Hopefully

      Hopefully means "in a way that is full of hope," as in I sat in my seat hopefully, waiting to see if I had won the prize. It does not mean "I hope."
    21. All of a sudden

      not all of the sudden.
    22. Everyday is an adjective, every day describes time.

      Finding a bottle cap is an everyday occurrence, but it isn't every day that you find one made of gold.
    23. All right is two words.

      You wouldn't say alwrong, so why would you say alright?
    24. A lot is two words.

      You wouldn't say alittle, so why would you say alot?
    25. Yea is the opposite of nay, and means that you agree to a vote.

      Yay is an expression of joy. Yeah is slang for yes.
    26. Less/fewer

      Use fewer with things that you can count (or can make plural), less for those you can’t count or make plural. I should eat less food, but I should eat fewer potato chips.
    27. Aaaahhhh! — This is what you say when the doctor checks your throat.

      It is NOT a scream. If you want your character to scream, say something like: “Mary screamed in terror and hid behind the door.”
      1. In general, do not put sound effects in place of description.

    28. Itch/scratch — When your skin itches, you scratch it.

      Itch is not something a person can do on purpose; it is something a body part does all by itself.

    29. Nowadays, not now days.

    30. Unique

      If something is unique, it is one of a kind. It cannot be very unique or somewhat unique.

    31. Sank/sunk

      Sank is the past tense of sink. Sunk is only used with a helping verb: "The ship had sunk."
    32. Number / Amount

      Amount is used for things you can't count, such as love, beauty, courage. Number is used for things you can count, such as flowers, candy, and houses.
    33. That / Which 

      If the sentence needs the part after the conjunction, use that. If it doesn't, use which.
      • Our room, which has three bookcases, is next to the Music Room.
      • Our room that has three bookcases is next to the Music Room. 
      In the first one, the part after which is not essential -- it's just extra information. In the second one, it's telling you which of the rooms we are talking about.