While I try to help out with grammar in these weekly Grammar Rules posts, I don't think of myself as part of the grammar police. However, everyone has their pet peeves, and one of mine is related to the misuse of principal and principle. One could be the top position at a school; the other could be related to a person moral code.
So let's look at when to use principal and principle for these instances and others.
Principal vs. Principle
Principal can be used as a noun or adjective. As an adjective, it means that someone or something is the most important or influential. As a noun, it's used a few ways. The first one is to describe a person who is in charge or who has responsibility over others. It can also mean a person who commits a crime. A few other meanings are used in matters of finance and construction.
Principle, on the other hand, is a noun that often relates to a law or moral code and/or a person who upholds those laws or moral code. Principle can also refer to the primary source of something (like Sigmund Freud's pleasure principle mentioned in the following song by Janet Jackson).
Here are a couple examples:
Correct: She is the principal of the high school.
Incorrect: She is the principle of the high school.
Correct: He really lacks the principles to be trusted.
Incorrect: He really lacks the principals to be trusted.
While principal can be used as an adjective and noun, principle can only be used as a noun (though principled can be used as an adjective). Also, principal can refer to a person, while principle can only refer to something a person has.
So here's how I keep them straight: I use the "pal" in "principal" to remember it can refer to the leader of a school or organization, while the other one refers to ideals such a person may have.
No matter what type of writing you do, mastering the fundamentals of grammar and mechanics is an important first step to having a successful writing career.