principal vs important

principal is a noun but important is not a noun.

Word	Noun	Adjective	Verb	Adverb
principal	Yes	Yes	No	No
important	No	Yes	No	No

principal (noun)	important (noun)
the educator who has executive authority for a school
an actor who plays a principal role
the major party to a financial transaction at a stock exchange; buys and sells for his own account
(criminal law) any person involved in a criminal offense, regardless of whether the person profits from such involvement
the original amount of a debt on which interest is calculated
capital as contrasted with the income derived from it

principal (adjective)	important (adjective)
most important element	of great significance or value
	of extreme importance; vital to the resolution of a crisis
	important in effect or meaning
	having or suggesting a consciousness of high position
	having authority or ascendancy or influence