Principal: main, or a person in charge
Principal (ends in -pal) has two common uses: as an adjective meaning "main" or "most important" ("the principal reason"), and as a noun for a person in charge, like a school principal, or the original amount of a loan (before interest): "she paid off the principal."
Principle: a rule or belief
Principle (ends in -ple) is a noun meaning a fundamental rule, law, or belief: "honesty is one of his core principles." "the principle of supply and demand."
The trick
The school principal is your pal (friend) — principal ends in "-pal." Principle ends in "-ple," like "ruLE" — think of a rule or law.
Examples
Principal: "the principal cause of the delay," "she called the school principal," "pay down the loan principal."
Principle: "it's against my principles," "the basic principles of physics."
Frequently asked questions
What's the classic trick for principal vs. principle?
The school "principal" is your "pal" — both end in -pal. "Principle" ends in -ple, like "rule."
Is "principal" always about a school?
No — it can also mean "main/most important" (adjective) or the original amount of money in a loan or investment.
"Guiding principles" — principal or principle?
"Principles," because it refers to fundamental beliefs or rules, not a person or amount.
Can "principal" be an adjective?
Yes: "the principal reason," "the principal investigator" — meaning "main" or "primary."