What is the plural form of i.e. (that is)?
Is it that the same i.e. is also used in its plural form? But I have not yet come across such usage.
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As far as I know, like the equivalent English expressions "that is" or "that is to say", "i.e." has no plural form, and can be used before or after a plural noun phrase. The Collins Dictionary entry for "i.e." gives the following example:
...strategic points–i.e. airports or military bases.
"I.e" originated as an abbreviation of the Latin phrase "id est". I don't know the rules for using this expression in Latin (if you're interested in learning, I'd recommend posting a question on latin.stackexchange.com), but I've found at least one example of it being used between plural noun phrases, "ubi centriae id est musculi brachiolares sunt" (Vegetius 1.25.5, mentioned in Pelagonius and Latin Veterinary Terminology in the Roman Empire, by James Noel Adams, 1995).
There is no need for the plural form of i.e. (id est), as it is normally used in English, because the antecedent of id in it is the point (claim, proposition, fact, state of affairs) that precedes it (which is then clarified, elucidated, or explicated in what follows it). That point is regarded as one item, even if it concerns a number of distinct things.
Note that the phrase that is, when used for the same purpose, behaves the same way. For example, one can say:
The course is open to sophomores, juniors, and seniors, that is, to all students except freshmen.
“That” in this sentence, does not refer to the multitude of sophomores etc., but to the one fact that the course is open to them. Analogously, one can say:
The course is open to sophomores, juniors, and seniors, i.e., to all students except freshmen.
The phrase id est in it is in singular, just like that is in the preceding example.