stern vs severe

stern is a noun but severe is not a noun.

Word	Noun	Adjective	Verb	Adverb
stern	Yes	Yes	No	No
severe	No	Yes	No	No

As adjectives, stern and severe are synonyms defined as:

Other synonyms of stern include austere, stark.

stern (noun)	severe (noun)
the rear part of a ship
the fleshy part of the human body that you sit on

stern (adjective)	severe (adjective)
severe and unremitting in making demands	causing fear or anxiety by threatening great harm
not to be placated or appeased or moved by entreaty	intensely or extremely bad or unpleasant in degree or quality
of a stern or strict bearing or demeanor; forbidding in aspect	severely simple
severely simple	very strong or vigorous
	very bad in degree or extent
	unsparing and uncompromising in discipline or judgment