U.S. vs first-year

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

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

U.S. (noun)	first-year (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

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