In lang=en terms the difference between substitute and spare
is that
substitute is to serve as a replacement (for someone or something) while
spare is (
to give up) to deprive oneself of, as by being frugal; to do without; to dispense with; to give up; to part with.
As verbs the difference between substitute and spare
is that
substitute is to use in place of something else, with the same function while
spare is to show mercy.
As nouns the difference between substitute and spare
is that
substitute is a replacement or stand-in for something that achieves a similar result or purpose while
spare is the act of sparing; moderation; restraint.
As an adjective spare is
scanty; not abundant or plentiful.