Here’s a goof I see all the time. It doesn’t help that the two words are pronounced the same and only differ by two letters, but they have very different meanings. The two meanings can even have serious legal significance, so ignore them at your peril.
Principal as a noun has two meanings: an administrator, e.g. principal of a school; or a sum of money, e.g. principal of a loan.
Principal as an adjective means “primary” or “most important,” e.g. “The principal reason we are firing you is your refusal to wear pants.”
Principle is a noun, meaning “rule” or “maxim,” e.g. “I will not wear pants, because to do so would violate my principles.”
A guide some of us learned as kids is to think of the principal as your pal. That only covers one possible definition, but it’s a start.