Principal, as an adjective, means foremost. As a noun, it is the head of a school or the amount of money borrowed.
- Who are the principal developers of the new hardware product?
- The middle school is getting a new principal for the fall.
- The mortgage payment includes both principal and interest.
Principle is a noun that refers to law or personal conviction.
- It works on the principle that warm air rises.
- It is against our basic principles to make such a statement.