father-in-law vs in-law

father-in-law and in-law both are nouns.

Word	Noun	Adjective	Verb	Adverb
father-in-law	Yes	No	No	No
in-law	Yes	No	No	No

As nouns, in-law is a hypernym of father-in-law; that is, in-law is a word with a broader meaning than father-in-law:

Other hypernyms of father-in-law include begetter, father, male parent, relative-in-law.

father-in-law (noun)	in-law (noun)
the father of your spouse	a relative by marriage