Here the word master is, so to say, undistributed. It may belong to either clause of the sentence, i.e., the whole sentence may be divided into
Either—it is I your master—
Or—your master who commands you.
This is the first point to observe. The next is that the verb in the second clause (command or commands) is governed, not by either the personal pronoun or the substantive, but by the relative, i.e., in the particular case before us, not by either I or master, but by who.
And this brings us to the following question—with which of the two antecedents does the relative agree? with I or with master?
This may be answered by the two following rules;—
Rule 1.—When the two antecedents are in the
same proposition, the relative agrees with the first. Thus—
1. It is I your master—
2. Who command you.