mental test vs sub-test

mental test and sub-test both are nouns.

Word	Noun	Adjective	Verb	Adverb
mental test	Yes	No	No	No
sub-test	Yes	No	No	No

As nouns, sub-test is a hyponym of mental test; that is, sub-test is a word with a more specific, narrower meaning than mental test:

mental test: any standardized procedure for measuring sensitivity or memory or intelligence or aptitude or personality etc
sub-test: one of a battery of related tests

Other hyponyms of mental test include intelligence test, IQ test, personality test.

mental test (noun)	sub-test (noun)
any standardized procedure for measuring sensitivity or memory or intelligence or aptitude or personality etc	one of a battery of related tests