main vs important

main is a noun but important is not a noun.

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

main (noun)	important (noun)
a principal pipe in a system that distributes water or gas or electricity or that collects sewage
any very large body of (salt) water

main (adjective)	important (adjective)
(of a clause) capable of standing syntactically alone as a complete sentence	of great significance or value
most important element	of extreme importance; vital to the resolution of a crisis
of force; of the greatest possible intensity	important in effect or meaning
	having or suggesting a consciousness of high position
	having authority or ascendancy or influence