I recently received an email asking about the expression woe is me, namely what the plural would be and why it’s not woe am I. Though the phrase may strike modern speakers as bizarre if not downright ungrammatical, there’s actually a fairly straightforward explanation: it’s an archaic dative expression. Strange as it may seem, the correct form really is woe is me, not woe am I or woe is I, and the first-person plural would simply be woe is us. I’ll explain why.
Today English only has three cases—nominative (or subjective), objective, and genitive (or possessive)—and these cases only apply to personal pronouns and who. Old English, on the other hand, had four cases (and vestiges of a fifth), and they applied to all nouns, pronouns, and adjectives. Among these four were two different cases for objects: accusative and dative. (The forms that we now think of simply as object pronouns actually descend from the dative pronouns, though they now cover the functions of both the accusative and dative.) These correspond roughly to direct and indirect objects, respectively, though they could be used in other ways too.
For instance, some prepositions took accusative objects, and some took dative objects (and some took either depending on the meaning). Nouns and pronouns in the accusative and dative cases could also be used in ways that seem strange to modern speakers. The dative, for example, could be used in places where we would normally use to and a pronoun. In some constructions we still have the choice between a pronoun or to and a pronoun—think of how you can say either I gave her the ball or I gave the ball to her—but in Old English you could do this to a much greater degree.
In the phrase woe is me, woe is the subject and me is a dative object, something that isn’t allowed in English today. It really means woe is to me. Today the phrase woe is me is pretty fixed, but some past variations on the phrase make the meaning a little clearer. Sometimes it was used with a verb, and sometimes woe was simply followed by a noun or prepositional phrase. In the King James Bible, we find “If I be wicked, woe unto me” (Job 10:15). One example from Old English reads, “Wa biþ þonne þæm mannum” (woe be then [to] those men).
So “woe is I” is not simply a fancy or archaic way of saying “I am woe” and is thus not parallel to constructions like “it is I”, where the nominative form is usually prescribed and the objective form is proscribed. In “woe is me”, “me” is not a subject complement (also known as a predicative complement) but a type of dative construction.
Thus the singular is is always correct, because it agrees with the singular mass noun woe. And though we don’t have distinct dative pronouns anymore, you can still use any pronoun in the object case, so woe is us would also be correct.