Principal means primary or most important.
The ability to communicate effectively is the principal requirement for the administrative position.
Principal is also used, in its noun form, to refer to the chief administrator of a school. (An easy way to remember the spelling is to think of the school principal as your pal.)
Principle is a doctrine or rule of conduct.
The United States was founded on Biblical principles.
Another common use of principle is with the phrase “on principle.”
The victim sued for damages, not for the money, but on the basis of principle.