important vs main

important is not a noun while main is a noun.

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

important (noun)	main (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

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