In English, Greek, and Hebrew a subject must agree with its verb in gender and number; e.g. a plural verb requires a plural subject; a singular verb requires a singular subject.
See principle 7 and principle 21.
A subject must agree with its verb in number. video
An attributive adjective must agree with whatever it is modifying in gender, number, and definiteness.
A predicate adjective precedes the noun it modifies. It must agree with this noun in gender, number, but not definiteness.