A Bit About Pronouns

The pronouns that and which are often interchanged, and sometimes used incorrectly. That is a demonstrative pronoun. It defines and stands for the noun it replaces:

The blue chair in the corner--that one is most comfortable.

Which is considered a relative pronoun. It highlights attributes, but not necessarily the object:

The blue chair, which is most comfortable, is in the corner.

Which will usually indicate a clause and be offset with commas.

The spoken language used by most people is quite informal and doesn't always distinguish between that and which. When writing dialogue, not using the proper pronoun would be acceptable for realistic speech patterns. In narration, however, be careful. Switch out those misplaced whiches to make a more polished work.