principal, principle

Principal can mean main or most important. As a noun, it can name someone in a leading role or an amount of money before interest. Principle names a belief, a guiding rule, or a basic truth.

Main: Her principal reason for moving was to live near the sea.

A guiding belief: Kindness is one of his principles.

Try replacing principal with main when it describes something. A principle is an idea you believe in or a rule that explains how something works.