Basic reason is enough to know that when only one person is spoken of, singular personal pronouns are employed, while when a plural number of persons are in view, this is grammatically reflected by the usage of plural personal pronouns. The inspired scriptures are careful in this respect, since they are in every respect accurate and without error. Every letter of the holy scriptures, the Lord taught, is of value, importance, and significance (Matt. 5:17-18, Matt. 22:32). Thus we can and must learn even from the fine details of scripture, not least of which is the sort of pronouns it uses; ordinarily when scripture uses a singular pronoun, it is intentionally communicating to us that a single person is in view, while it denotes multiple persons by using plural personal pronouns.
This basic logic disproves the false claims made by semi-modalists that whenever God is spoken of as a person, “God” ought to be taken as signifying the Trinity, although nothing in the passage would suggest such a reading. Far from giving us a reason to think this, scripture refutes this attempt to import heretical presuppositions into scripture by frequently using singular personal pronouns for God, thus showing that the subject referred to by the term “God” -usually the Father- is a single person, not a plurality of persons.
Yet, the resilience of this error shows itself, when, instead of conceding their error in the face of conviction by both basic logic and the holy scriptures, the semi-modalists instead attempt to turn things on their head once more by declaring that singular personal pronouns must be used for a plural number of persons of the Trinity. They argue this point by means of further extra-biblical theories, among which, is the notion that all three persons of the Trinity always act inseparably with one united action; thus since the action is singular, the personal pronoun must also be singular.
We might first observe in response to this that nowhere does scripture say that the Father cannot act without the Son and Spirit, although they are correct in noting that the Son acts only according to the will of the Father, and at the instigation of the Father. Frequently we see God work through the Son and Holy Spirit. Yet to say that this is always so is scripturally unwarranted.
Further, and more definitively, we may respond by simply noting that the pronouns under discussion are not pronouns used to refer to an action- which may perhaps be singular, although performed by multiple persons in unity- but rather refer to the actors themselves Who perform the action. In such a scenario then as that three persons together act with one action, although the action is then singular, and may be referred to as such, yet the actors remain plural, being three persons, and the way they are referred to must logically remain plural personal pronouns. To use a singular personal pronoun from Them would not so much indicate a unity of action, but would cause the three persons to appear to be a single actor and a single person.
So much then, for the attempt to justify using singular personal pronouns for a plural number of persons. When the semi-modalists do so, though they may object and try in vain to find some justification for it, yet it is proven that they, by their use of a singular pronoun for three persons, present the three persons of the Trinity as a single individual, in true modalist fashion.
Finally it is worthwhile to note that those who insist that singular personal pronouns ought to be used for the persons of the Trinity together, charge the scriptures with erring, since the scriptures, as one would expect, and as we have said above, use plural pronouns for multiple persons of the Trinity together:
“Fall on us and hide us from the presence of Him who sits on the throne, and from the wrath of the Lamb; 17 for the great day of their wrath has come, and who is able to stand?” (Rev. 6:16-17 NASB)
“Jesus answered and said to him, “If anyone loves Me, he will keep My word; and My Father will love him, and We will come to him and make Our home with him.” (John 14:23 NASB)
The home that is made is one; the day of wrath is one; yet, God and His Son are two persons, and so scripture speaks in accordance with that truth by using plural, not singular, personal pronouns for both persons together.