Now the predicate need agree with the subject in case only.
1. It has no necessary concord in gender—she is a man in courage—he is a woman in effeminacy—it is a girl.
2. It has no necessary concord in number—sin is the wages of death—it is these that do the mischief.
3. It has no necessary concord in person—I am he whom you mean.
4. It has, however, a necessary concord in case. Nothing but a nominative case can, by itself, constitute a term of either kind—subject or predicate. Hence, both terms must be in the nominative, and, consequently, both in the same case. Expressions like this is for me are elliptic. The logical expression is this is a thing for me.
Rule.—The predicate must be of the same case with its subject.
Hence—The copula instead of determining[[60]] a case expresses a concord.
Rule 1.—All words connected with a nominative case by the copula (i.e., the so-called verb-substantive) must be nominative.—It is I; I am safe.
Rule 2.—All words in apposition with a word so connected must be nominative.—It is difficult to illustrate this from the English language from our want of inflexions. In Latin, however, we say vocor Johannes=I am called John, not vocor Johannem. Here the logical equivalent is ego sum vocatus Johannes—where—