substitute vs alternate

substitute and alternate both are nouns.

Word	Noun	Adjective	Verb	Adverb
substitute	Yes	Yes	Yes	No
alternate	Yes	Yes	Yes	No

As adjectives, substitute and alternate are synonyms defined as:

Other synonyms of substitute include alternative.

As nouns, alternate is a hyponym of substitute; that is, alternate is a word with a more specific, narrower meaning than substitute:

substitute: someone who takes the place of another (as when things get dangerous or difficult)
alternate: someone who takes the place of another person

Other hyponyms of substitute include locum, locum tenens, double, replacement, surrogate.

substitute (noun)	alternate (noun)
a person or thing that takes or can take the place of another	someone who takes the place of another person
someone who takes the place of another (as when things get dangerous or difficult)
an athlete who plays only when a starter on the team is replaced

substitute (adjective)	alternate (adjective)
artificial and inferior	of leaves and branches etc; first on one side and then on the other in two ranks along an axis; not paired
serving or used in place of another	serving or used in place of another
capable of substituting in any of several positions on a team	occurring by turns; first one and then the other
	every second one of a series

substitute (verb)	alternate (verb)
put in the place of another; switch seemingly equivalent items	do something in turns
be a substitute	go back and forth; swing back and forth between two states or conditions
act as a substitute	reverse (a direction, attitude, or course of action)
	be an understudy or alternate for a role
	exchange people temporarily to fulfill certain jobs and functions