alternate vs substitute

alternate and substitute both are nouns.

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

As adjectives, alternate and substitute are synonyms defined as:

Other synonyms of alternate include alternative.

As nouns, substitute is a hypernym of alternate; that is, substitute is a word with a broader meaning than alternate:

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

Other hypernyms of alternate include backup, backup man, fill-in, relief, reliever, stand-in.

alternate (noun)	substitute (noun)
someone who takes the place of another person	a person or thing that takes or can take the place of another
	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

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

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