Objection 1. It seems that this is not the proper form of this sacrament: "This is My body." For the effect of a sacrament ought to be expressed in its form. But the effect of the consecration of the bread is the change of the substance of the bread into the body of Christ, and this is better expressed by the word "becomes" than by "is." Therefore, in the form of the consecration we ought to say: "This becomes My body."
Objection 2. Further, Ambrose says (De Sacram. iv), "Christ's words consecrate this sacrament. What word of Christ? This word, whereby all things are made. The Lord commanded, and the heavens and earth were made. " Therefore, it would be a more proper form of this sacrament if the imperative mood were employed, so as to say: "Be this My body."
Objection 3. Further, that which is changed is implied in the subject of this phrase, just as the term of the change is implied in the predicate. But just as that into which the change is made is something determinate, for the change is into nothing else but the body of Christ, so also that which is converted is determinate, since only bread is converted into the body of Christ. Therefore, as a noun is inserted on the part of the predicate, so also should a noun be inserted in the subject, so that it be said: "This bread is My body."
Objection 4. Further, just as the term of the change is determinate in nature, because it is a body, so also is it determinate in person. Consequently, in order to determine the person, it ought to be said: "This is the body of Christ."
Objection 5. Further, nothing ought to be inserted in the form except what is substantial to it. Consequently, the conjunction "for" is improperly added in some books, since it does not belong to the substance of the form.
On the contrary, our Lord used this form in consecrating, as is evident from Matthew 26:26.
I answer that, This is the proper form for the consecration of the bread. For it was said (1) that this consecration consists in changing the substance of bread into the body of Christ. Now the form of a sacrament ought to denote what is done in the sacrament. Consequently the form for the consecration of the bread ought to signify the actual conversion of the bread into the body of Christ. And herein are three things to be considered: namely, the actual conversion, the term "whence," and the term "whereunto."
Now the conversion can be considered in two ways: first, in "becoming," secondly, in "being." But the conversion ought not to be signified in this form as in "becoming," but as in "being." First, because such conversion is not successive, as was said above (Question 75, Article 7), but instantaneous; and in such changes the "becoming" is nothing else than the "being." Secondly, because the sacramental forms bear the same relation to the signification of the sacramental effect as artificial forms to the representation of the effect of art. Now an artificial form is the likeness of the ultimate effect, on which the artist's intention is fixed;. just as the art-form in the builder's mind is principally the form of the house constructed, and secondarily of the constructing. Accordingly, in this form also the conversion ought to be expressed as in "being," to which the intention is referred.
And since the conversion is expressed in this form as in "being," it is necessary for the extremes of the conversion to be signified as they exist in the fact of conversion. But then the term "whereunto" has the proper nature of its own substance; whereas the term "whence" does not remain in its own substance, but only as to the accidents whereby it comes under the senses, and can be determined in relation to the senses. Hence the term "whence" of the conversion is conveniently expressed by the demonstrative pronoun, relative to the sensible accidents which continue; but the term "whereunto" is expressed by the noun signifying the nature of the thing which terminates the conversion, and this is Christ's entire body, and not merely His flesh; as was said above (76, 1, ad 2). Hence this form is most appropriate: "This is My body."
Reply to Objection 1. The ultimate effect of this conversion is not a "becoming" but a "being," as stated above, and consequently prominence should be given to this in the form.
Reply to Objection 2. God's word operated in the creation of things, and it is the same which operates in this consecration, yet each in different fashion: because here it operates effectively and sacramentally, that is, in virtue of its signification. And consequently the last effect of the consecration must needs be signified in this sentence by a substantive verb of the indicative mood and present time. But in the creation of things it worked merely effectively, and such efficiency is due to the command of His wisdom; and therefore in the creation of things the Lord's word is expressed by a verb in the imperative mood, as in Genesis 1:3: "Let there be light, and light was made."
Reply to Objection 3. The term "whence" does not retain the nature of its substance in the "being" of the conversion, as the term "whereunto" does. Therefore there is no parallel.
Reply to Objection 4. The pronoun "My," which implicitly points to the chief person, i.e. the person of the speaker, sufficiently indicates Christ's person, in Whose person these words are uttered, as stated above (Article 1).
Reply to Objection 5. The conjunction "for" is set in this form according to the custom of the Roman Church, who derived it from Peter the Apostle; and this on account of the sequence with the words preceding: and therefore it is not part of the form, just as the words preceding the form are not.
Source: Summa Theologica (New Advent)