[§ 553]. The verb must agree with its subject in person, I walk, not I walks: he walks, not he walk.
It must also agree with it in number,—we walk, not we walks: he walks, not he walk.
Clear as these rules are, they require some expansion before they become sufficient to solve all the doubtful points of English syntax connected with the concord of the verb.
A. It is I, your master, who command you. Query? would it is I, your master, who commands you, be correct? This is an example of a disputed point of concord in respect to the person of the verb.
B. The wages of sin is death. Query? would the wages of sin are death be correct? This is an example of a disputed point of concord in respect to the number of the verb.
[§ 554]. In respect to the concord of person the following rules will carry us through a portion of the difficulties.
Rule.—In sentences, where there is but one proposition, when a noun and a pronoun of different persons are in apposition, the verb agrees with the first of them,—I, your master, command you (not commands): your master, I, commands you (not command).
To understand the nature of the difficulty, it is 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.