principal - principle
Principal can be an adjective or a noun.
The principal thing or person in a group is the most important one.
His principal interest in life was money.
The principal character in the film was played by John Hurt.
The principal of a school or college is the person in charge of it.
The teacher sent me to the principal's office.
Lodge was Principal of Birmingham University.
Principle is always a noun. A principle is a general rule that someone's behaviour or ideas are based on.
She did not eat meat because it was against her principles.
We follow the principle that everyone should be treated equally.