I'm writing this post because for the thousandth time since I've been in the writing consulting business, I fielded the question, "What's the difference between principal and principle?" Why not put my response here.
I've heard time and again that you can remember principal because the principal of the school is your pal. Not very helpful if you want to know about the principal of a company, the principal of a loan, the principal reason for the conflict, or your principal principle.
The best way to remember the difference between this homophone pair is to think of principal as main, as in "The principal is the main person of a school or company, the main part of a loan, the main reason, and your main principle." The principle is a rule or belief.