As a noun, principal generally means main or head person, such as the principal of a school. (Some people remember this meaning using the memory device, “the principal is our pal.”) As a noun, it can also mean a capital sum of money; a perpetrator or aider and abettor to a crime; or one who employs another (an agent) to act for him. When used as an adjective, principal means “main” or “primary,” such as the principal finding in a study.
On the other hand, principle is a noun that means a rule, tenet, or basic truth, such as the principle of gravity.
Thus, use principal as a noun to refer to important people and as an adjective to signify someone or something important. In contrast, use principle, a noun, when you mean a rule or doctrine.
- The principal of the high school led the students in singing the national anthem. (principal as a noun meaning the main or head person)
- The principal issue in this case is whether the defendant waived his right to trial by jury. (principal as an adjective meaning main or primary)
- Freedom and justice are the principles upon which our country was founded. (principle meaning rule, tenet or basic truth)