Q: I try not to pay much attention to grammar in songs, knowing it gets sacrificed for rhythm and such. But hymns in church are almost always in proper English, which is why I did a double take to see: “O come, O come, Emmanuel, / And ransom captive Israel, / That mourns in lonely exile here / Until the Son of God appear.” Is it just me, or should the last word be “appears”?
A: The use of “appear” in the hymn “O come, O come Emmanuel” (John Mason Neal’s 19th-century translation of the Latin text Veni, veni, Emmanuel) is an archaic use of the subjunctive mood.
The meaning of the last line you cited is “Until the Son of God [should, or happens to] appear.”
We no longer use the subjunctive in a sentence like this. We would use the indicative mood (“appears”). However, an echo of that old usage survives in the expression “until death do us part.”
The subjunctive mood is used to express something hypothetical: something wished, imagined, demanded, and so on.
It was once used much more often than it is today. These days, English speakers use the subjunctive mood (instead of the normal indicative mood) for only three purposes:
(1) To express a wish: “I wish I were thinner.” [Not: “I wish I was thinner.”]
(2) To express an “if” statement about a condition that’s contrary to fact: “If I were a rich man …” [Not: “If I was a rich man …”]
(3) To express that something is being asked, demanded, ordered, suggested, and so on: “I suggest he find a job.” [Not: “I suggest he finds a job.”]
However, some older vestiges of the subjunctive survive in expressions like “God forbid” [not “God forbids”]; “Long live the Queen” [not “Long lives the Queen”]; “so be it” [not “so is it”]; and “come what may” [not “comes what may”].
We also use it in sentences like “I hurried, lest I be late,” and “He loves art, whether it be painting, sculpture, or music,” and “Come February, the mortgage will be paid off.”
If you’d like to read more, I wrote a post last year about the modern use of the subjunctive in English.