first-year vs U.S.

first-year is not a noun while U.S. is a noun.

Word	Noun	Adjective	Verb	Adverb
first-year	No	Yes	No	No
U.S.	Yes	No	No	No

first-year (noun)	U.S. (noun)
	the executive and legislative and judicial branches of the federal government of the United States
	North American republic containing 50 states - 48 conterminous states in North America plus Alaska in northwest North America and the Hawaiian Islands in the Pacific Ocean; achieved independence in 1776

first-year (adjective)	U.S. (adjective)
used of a person in the first year of an experience (especially in United States high school or college)