In Spanish, you usually add -s to form plurals, to form singulars you "undo" that and remove the suffix -s.
In Spanish the subject and verb of a sentence must have the same grammatical person and the same grammatical number. Thus, after changing the endings with or without -s (mostly) we need to change the verb form.
"Son" is the 3rd plural person verb form of the present tense of ser (to be). "Es" is the 3rd singular person verb form.
"Es" is the 3rd singular person verb form of the present tense of ser (to be). "Son" is the 3rd plural person verb form.
"Son" is the 3rd plural person verb form of the present tense of ser (to be). "Es" is the 3rd singular person verb form.
"Son" is the 3rd plural person verb form of the present tense of ser (to be). "Es" is the 3rd singular person verb form