You might know a priest like the one I have in mind: is reverent during Mass, gives competent homilies, and is faithful to the rubrics—but with an exception or two, just enough to annoy.
The priest I’m thinking of has a habit of substituting "The Lord is with you" for "The Lord be with you." An inconsequential thing, to some people, but not to those of us who remember that English has a mood called the subjunctive, even if it is used far more rarely than in, say, Italian, where the subjunctive intrudes into nearly every other sentence.
The priest I’m thinking of is not a native speaker of English. I do not know whether the language of his upbringing (not a European language) has the subjunctive. To him, "The Lord is with you" and "The Lord be with you" might seem to mean the same, with the latter being in what he perceives to be archaic language. If that is what he thinks, it would mitigate the substitution of one sentence for the other, but it would not excuse it, since a priest is required to celebrate Mass strictly by the book. Every deviation, every independent "improvement," is forbidden because the Mass is not the private prayer of the priest. It is the public prayer of the Church, and so it has tightly fixed formulations.
I cannot say whether the subjunctive is taught nowadays in public schools. I hesitate to ask an English teacher because I fear the answer might be: "What’s the subjunctive?" If it is not taught, it ought to be. If it is taught, it ought to be taught more concertedly, because it is an important part of the language. Through it we can express things that cannot be easily expressed otherwise.
No doubt many Catholics, priests and laymen alike, think that "The Lord is with you" and "The Lord be with you" are equivalent, but that is wrong. The first sentence states a fact, the second a hope. When we say "The Lord be with you," we are saying something like "I hope the Lord will be with you (he may or may not be with you at the moment—I cannot tell)." When we say "The Lord is with you," we are saying something like "I know for certain that the Lord already is with you."
In the Gospels our Lord admonishes us "not to judge lest you be judged," meaning that we should not presume to judge the spiritual state of those around us. The one we take to be spiritually lost might be fit for heaven at this very moment; the one we take to be saintly might be on his way to perdition. Similarly, we can hope that the Lord is with someone, but we cannot know indubitably that he is. The person may have turned him away through sin or just through lack of interest in him.
Even after someone has returned from Communion, we cannot say for sure, "That man now is in the state of grace." If he took Communion while in the state of mortal sin, he remains in the state of mortal sin when he returns to his pew. Even at Communion time we must fall into a kind of subjunctive—hoping that something is so without being able to know for certain that it is so.