Principle or Principal?
Exemple : ‘’I refused, on principle, to let him enter.’’
These two words are very easily confused though they have very different meanings. "Principle" is a noun which means "a general rule used as a basis for a particular theory". It can also be used to denote a rule or belief about what is right and wrong which effects the way in which a person behaves.
Exemple : ‘’The basic principal of the class is to improve your English grammar.’’
"Principal" is most commonly used as an adjective meaning "main" or "most important". It can also be used as a noun, particularly in American English, to denote the head of a school or organisation.