What's the "een" in Halloween?
late 14c., "mutual conformity of things;" c.1400, "mutual understanding" (among persons), also (of things) "mutual conformity," from Old French agrement, noun of action from agreer "to please" (see agree).
The subject and verb of a clause or simple sentence must agree in person, as in “He is a boy.” The subject, he, and the verb, is, are both in the third person. The subject and verb also must agree in number, as in “We are girls.” The subject, we, and the verb, are, are both plural.
Nouns and pronouns must also agree in number, person, and gender as in “Every boy must mind his manners.” The noun boy and the pronoun his are both singular, both in the third person, and both masculine.