necessary to remember that subjects may be extremely complex as well as perfectly simple; and that a complex subject may contain, at one and the same time, a noun substantive and a pronoun,—I, the keeper; he, the merchant, &c.
Now all noun-substantives are naturally of the third person—John speaks, the men run, the commander gives orders. Consequently the verb is of the third person also.
But the pronoun with which such a noun-substantive may be placed in apposition, may be a pronoun of either person, the first or second: I or thou—I the commander—thou the commander.—In this case the construction requires consideration. With which does the verb agree? with the substantive which requires a third person? or with the pronoun which requires a first or second?
Undoubtedly the idea which comes first is the leading idea; and, undoubtedly, the idea which explains, qualifies, or defines it, is the subordinate idea: and, undoubtedly, it is the leading idea which determines the construction of the verb. We may illustrate this from the analogy of a similar construction in respect to number—a man with a horse and a gig meets me on the road. Here the ideas are three; nevertheless the verb is singular. No addition of subordinate elements interferes with the construction that is determined by the leading idea. In the expression I, your master, the ideas are two; viz., the idea expressed by I, and the idea expressed by master. Nevertheless, as the one only explains or defines the other, the construction is the same as if the idea were single. Your master, I, is in the same condition. The general statement is made concerning the master, and it is intended to say what he does. The word I merely
defines the expression by stating who the master is. Of the two expressions the latter is the awkwardest. The construction, however, is the same for both.
From the analysis of the structure of complex subjects of the kind in question, combined with a rule concerning the position of the subject, which will soon be laid down, I believe that, for all single propositions, the foregoing rule is absolute.
Rule.—In all single propositions the verb agrees in person with the noun (whether substantive or pronoun) which comes first.
[§ 477]. But the expression it is I your master, who command (or commands) you, is not a single proposition. It is a sentence containing two propositions.
1. It is I.
2. Who commands you.