the difference between "which" and "that."

"Which" and "that" are known as "relative pronouns."  The term derives from the role the words play in relating parts of a sentence.  Compare the following sentences:

(1) Walk on the left side of the street until you reach the third house that is red.

  (2) Walk on the left side of the street until you reach the third house, which is red.

Do the two sentences above lead you to the same house? Not necessarily.

The first sentence leads you to the third RED house on the left side of the street.  This may be the third house on the left side of the street (if the first two are also red) or it may be the eighth house on the left side of the street, or the tenth house, etc.

The second sentence always leads you to the THIRD house on the left house on the street. This house happens to be red.

Notice that the first sentence employs the word "that" to indicate that the color RED is essential to identifying the house. The second sentence uses "which" to introduce a clause containing information that is NOT necessary to identify the house.

So, you use "that" when the information that follows is needed to identify the subject of the main clause and you use "which" when the information that follows is NOT needed to identify the subject of the main clause.  

             Now                when you see sentences containing "which" or "that" in Sentence                Correction, you will be equipped to handle them!