Plain is a related term of primary.
As adjectives the difference between plain and primary
is that
plain is while
primary is the first in a group or series.
As nouns the difference between plain and primary
is that
plain is (rare|poetic) a lamentation or
plain can be an expanse of land with relatively low relief while
primary is a primary election; a preliminary election to select a political candidate of a political party.
As verbs the difference between plain and primary
is that
plain is to lament, bewail or
plain can be (obsolete|transitive) to plane or level; to make plain or even on the surface while
primary is (us|intransitive) to take part in a primary election.
As an adverb plain
is (colloquial) simply.