In a categorical proposition, the subject and predicate are connected by:

A categorical proposition links its subject and predicate through the copula, a form of the verb to be.

The copula is the joining word such as is, are, is not, or are not.

It asserts or denies the predicate class of the subject class and fixes the quality.

Without this connector the two terms would stand as isolated concepts, not a proposition.

This joining word carries the quality of the proposition, marking it affirmative or negative.