Q: Pat writes in her grammar book Woe Is I that it’s OK to say “that’s him” and “that’s her” in all but the most formal writing. I don’t know how this could be true. The pronouns are nominative case. This means they should be “that’s he” and “that’s she.”
A: You’ve raised an issue that’s been argued among English grammarians for the last 250 years.
The question: After the use of the verb “be,” should a subject or an object pronoun be used?
In other words, must we always say, “It is he”? Or is “It is him” just as good?
The thinking among modern grammarians and writers on usage is that a subject pronoun (“That is he,” “This is I,” etc.) is appropriate in formal English.
But an object, these language writers say, is fine in informal English (“That’s him,” “This is me”).
Merriam-Webster’s Dictionary of English Usage has an excellent discussion of this in its entry for “it’s me.”
“The venerable argument over the nominative versus the objective case after the verb to be is a memorable part of our linguistic heritage,” M-W says.
The arguing began in the 1760s, with the two great warring camps led by Robert Lowth (nominative) versus Joseph Priestly (objective).
The remnants of those 18th-century arguments are still with us. But today “the grounds have been shifted,” as M-W puts it.
Now the difference between the two camps is no longer one of correct versus incorrect or standard versus nonstandard, but of “formal versus colloquial styles.”
“So instead of the old choice between right and wrong,” M-W explains, “we are now choosing a style; it is a choice that is much closer to the reality of usage than the old one was.”
It’s also worth mentioning that this “nominative-after-be” rule is a convention of Latin grammar, a convention the isn’t even observed by all Latinate languages (witness c’est moi).
And why should English be expected to adhere to a convention of Latin grammar? After all, English is not Latinate but Germanic.
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