of import vs main

of import is not a noun while main is a noun.

Word	Noun	Adjective	Verb	Adverb
of import	No	Yes	No	No
main	Yes	Yes	No	No

of import (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

of import (adjective)	main (adjective)
of great significance or value	(of a clause) capable of standing syntactically alone as a complete sentence
	most important element
	of force; of the greatest possible intensity