Following is a list of words that people frequently confuse and misuse.  This list contains several words with which I have seen students in this class have problems.  I have linked each word with a definition or explanation and examples that will help you differentiate them so that you use these words correctly.

1.  affect/effect
A.  Generally, you would use affect when you want a verb:  How will the President's executive orders on prosecuting  people suspected of terrorism affect our civil liberties?

B.  Generally, you would use effect when you want a noun:  Many people fear the effect of the President's executive orders on prosecuting people suspected of terrorism will leave open the door to squashing legitimate political dissent.

C. Effect can be used as a verb when it means to cause, rather than to influence (affect).  For example:  The President's executive orders on prosecuting people suspected of terrorism will effect a change in the judicial process.

D.  Affect is rarely used as a noun, only when one is describing a symptom of a larger problem.  In this case, usually affect refers to a medical condition:  An affect of MS could be limping.

2.  its/it's
A.  Its is a possessive pronoun:  The horse kicked up its hind legs.
B.  It's is a contraction of it and is: It's time to do your homework.  You are saying:  It is time to do your homework.
A handy tip to make the distinction:  If you can substitute "it is" for the form you want, then use "it's".
You wouldn't say The horse kicked up it is legs!  Well, you might, but you'd sound pretty silly.

3. whose/ who's
Same deal as above:
A.  Whose is a possessive pronoun:  The teacher whose hat is on the table is Dr. Yang.
B.  Who's is a contraction of  who and is: Can you tell me who's going to the party?

4.  their, they're, and there

A.  Their is a possessive pronoun:  The students all earned 100s on their quizzes.
B.  They're is a contraction of they and are:  After getting 100s on their quizzes, they're going to celebrate.
C.  There refers to a place.  I'm going to join the students there for the celebration.
Consider that the word here is contained in the word there.  So, here means a place; there means a place.

5.  too, to, two

A.  Too is an adverb.  It means an excess.  Too many cooks spoil the broth.  Think of an excess amount, and in this word you have an excess of "o"s.
B.  To is a preposition.  It connects a noun to the rest of the sentence, often taking in words describing the noun:  I gave  high grades to my brilliant students in EN101. "To" connects "my brilliant students" to the rest of the sentence.  "Brilliant" and "my" describe students(a noun).
C.  If you don't know what two means, you're in BIG trouble!

6.  where, were

A. "Where" is an adverb.  Sometimes it functions as  subordinating conjunction.  It also refers to a place (note that "here" is also part of this word).  Where you go, I will follow.
B.  "Were" is a verb.  We were going to call you to join us.
So, don't use an adverb as an action word (or state of being) and don't use an action word (or state of being) to describe an adjective, adverb, or verb.

7.  lose, loose
A.  Lose is a verb that means either the opposite of win or to no longer have something you once possessed.
Did War Admiral lose the match race to Seabiscuit in the late 1930s?
Did someone lose Seabiscuit's halter in the barn?
B.  Loose means the opposite of tight.  You might think that the word is "loose" because it contains an extra "o."
If your pants are too loose they will fall down; so wear a belt.

8.  accept/except
A.  Accept means to bring in, to keep, to take in.
I accept the award that you gave me.
B.  Except means to leave out, to exclude:  remember the "ex" prefix means to go out or put out.
The teacher decided to except Leo from punishment because she knew he would never talk during study period.

9.  then/than
A.  then is an adverb or a conjunction.  It refers to when something happens next, following something else.
If you turn in your work on time, then you will get higher grades than someone who does not.
B. than  indicates a comparison, as you can see in the sentence above.
He is taller than me (Here a preposition).
I am smarter than he is (Here a conjunction).

10  have/of
A.  have is a verb that show possession or a helping (auxiliary) verb.
I have a plethora of hats (verb showing possession).
I have been known to wear two different hats in one day, but not at the same time (helping verb).
B.  of is a preposition.  You cannot use it as a verb!
I should have known that he was  the son of a governor.

11.  site, cite, sight
A.  site refers to a place; think of the word "situation":  I will be checking the progress of the new library when I go to the building site.

B.  cite means to note something.  Think of a Works Cited page where you note all the references you used in your paper.  Also think of a citation, which means to be noted for something good (commendation) or something bad (getting a speeding ticket).

C.  sight - certainly you must "see" what this one means!

12.  bias and prejudice are nouns.
I have a bias against tall people.

Do you have a prejudice against short people?

biased and prejdiced are participles and function as either descriptive words or part of a verb phrase.

He is a prejudiced person.

I was prejudiced by his friendship with a criminal.  (adjective, descriptive)

I wasn't biased by his friendly manner.

I wasn't prejudiced by his favors to my cat.