shadow vs darken

shadow is a noun but darken is not a noun.

Word	Noun	Adjective	Verb	Adverb
shadow	Yes	No	Yes	No
darken	No	No	Yes	No

As verbs, darken is a hypernym of shadow; that is, darken is a word with a broader meaning than shadow:

shadow (noun)	darken (noun)
refuge from danger or observation
something existing in perception only
an indication that something has been present
a premonition of something adverse
an unilluminated area
a spy employed to follow someone and report their movements
an inseparable companion
a dominating and pervasive presence
shade within clear boundaries

shadow (verb)	darken (verb)
follow, usually without the person's knowledge	become dark or darker
make appear small by comparison	make dark or darker
cast a shadow over	tarnish or stain