There was a push in the 1970s to make the parts of the mass in which the priest or deacon says, “The Lord be with you,” into a greeting, rather than a religious invocation. Thus, translators destroyed the translation of, “Et cum spiritu tuo,” by translating it as, “And also with you.” This was due in part to the “spirit” of Vatican II (note: I didn’t say Vatican II) which commends the use of the dynamic equivalence theory rather than the formal equivalence theory.
There is a massive difference in meaning. The phrase, “And also with you,” implies in no small part that what is happening is everyday, is common. The part in question is the “you,” since “you” is a word that is everyday, is common, and is not elevated either in breadth of vocabulary itself or in meaning. Anyone can say, “You too,” or, “And also with you,” and it means just that, the Lord be with you as well. Okay, kinda basic. But when you translate as, “And with your spirit,” not only are you more faithful to the Latin, but you have changed the meaning as well. I was listening to a priest comment on this very matter, and while his mode of delivery was rather apologetic in the sense that he sounded sorry that the Church was “forcing” such changes on the “poor people,” he did an excellent job of explaining it if you looked past that. He put forth the very real idea that by the laity responding, “And with your spirit,” they are making reference to the Lord being with the priest in the sense of Him being with the part of the priest’s soul that was permanently changed when he accepted Holy Orders. “And with your spirit” is a phrase that acknowledges the special character of the soul of the priest, and how it has a quality to it that is different from the laity’s souls, in the same sense that married people’s souls have a special character to them that makes them different from celibate priests’ souls. The phrase acknowledges this quality of the priest’s soul, which the old translation does not, at all. “And with your spirit” completes the priest’s invocation when he says, “The Lord be with you.”
Anyone with a rudimentary knowledge of English and who can make the most basic connections sees that “spiritu” and “spirit” are cognates.