Principle vs. Principal
Use principle when you're referring to a rule, something established as a standard, or the fundamental cause or origin of something.
Use principal when you're talking about a quantity of money or the person in charge of an organization, like a school. Principal can also be used as an adjective that means something is of the highest rank.
One way to remember the difference between principle and principal is to call upon TV's most famous principal: Mr. Belding from Saved by the Bell, who reminded his students that "the last three letters in principal spell pal." Aww.
Belding may not have always been on the ball with regard to what was happening in the hallowed halls of Bayside High, but he is useful for helping you remember that if you're talking about a person, you're talking about a principal.
"Doug," said Mrs. Glumball, "my principal concern with your final exam is that you wrote it all in crayon."
In this sentence, principal is used as an adjective that means that the crayon use is Mrs. Glumball's #1 concern about Doug's work.
Rich and Yvette are excited about their new home; they're less excited about paying down their mortgage principal.
Here, principal refers to a sum of money. Maybe Rich and Yvette should hold a cash-only housewarming party. Sure, it's tasteless, but a fat stack of $100 bills is far more useful than some wine charms or a magazine rack.
"For seventy-five years, my grandma has stuck to her principles and never, ever worn white after Labor Day."
In this fashion-conscious sentence, principles is the correct word because it refers to a set of rules that Grandma has set for herself.