mental strain vs stress

mental strain and stress both are nouns.

Word	Noun	Adjective	Verb	Adverb
mental strain	Yes	No	No	No
stress	Yes	No	Yes	No

As nouns, stress is a hyponym of mental strain; that is, stress is a word with a more specific, narrower meaning than mental strain:

Other hyponyms of mental strain include tenseness, tension.

mental strain (noun)	stress (noun)
(psychology) nervousness resulting from mental stress	the relative prominence of a syllable or musical note (especially with regard to stress or pitch)
	(physics) force that produces strain on a physical body
	(psychology) a state of mental or emotional strain or suspense
	difficulty that causes worry or emotional tension
	special emphasis attached to something

mental strain (verb)	stress (verb)
	to suffer from stress
	to stress, single out as important
	put stress on; utter with an accent
	test the limits of