"ADVICE" and "ADVISE"
"Advice" is a noun. "Advise" is a verb. Ex.: His advice was invaluable. Can you advise me on this issue?
"PRINCIPAL" and "PRINCIPLE"
"Principal" refers to a person in a position of authority or the primary part of something. "Principle" means a fundamental rule or belief. Ex.: The school principal addressed the students. The principal reason for our success is teamwork. Honesty is an important principle to uphold.