downcast vs grim

downcast is a noun but grim is not a noun.

Word	Noun	Adjective	Verb	Adverb
downcast	Yes	Yes	No	No
grim	No	Yes	No	No

As adjectives, downcast and grim are synonyms defined as:

Other synonyms of downcast include blue, depressed, dispirited, down, down in the mouth, downhearted, gloomy, low-spirited, low.

downcast (noun)	grim (noun)
a ventilation shaft through which air enters a mine

downcast (adjective)	grim (adjective)
filled with melancholy and despondency	shockingly repellent; inspiring horror
directed downward	filled with melancholy and despondency
	causing dejection
	not to be placated or appeased or moved by entreaty
	harshly ironic or sinister
	harshly uninviting or formidable in manner or appearance