“I” is singular, so why does it take plural verbs? Here's we say “I go to the store on Fridays” instead of “I goes to the store on Fridays.”
“Thou,” “Thee,” and “Ye” Were Replaced by “You”
The difference shows up clearly in the King James translation of the Bible, in John 8:32: “Ye shall know the truth, and the truth shall make you free.” These days, “you” does the job of both, and “ye” is only used by speakers who want to sound like old-timey sailors. So with this double dose of syncretism, instead of “thou,” “thee,” “ye,” and “you,” it’s all “you,” “you,” “you,” “you”! But we don’t say that we have four second-person pronouns and they’re all homonyms. We say that we just have one pronoun, “you,” which can be singular or plural, subjective or objective, as needed.
So how did English end up with so much syncretism? For plural verbs, it’s always been that way. Even back in Old English, the present tense for all three persons in the plural was the same form. To illustrate with an actual Old English verb, let’s take the verb that eventually developed into Modern English “deem.” The plural present tense form for all three persons was “demaþ” [day-mahth].
By the way, if you look at the transcript for this episode, you’ll see that the word ends with a character that looks like a lowercase “p” with a vertical part that goes up too far. That character is called “thorn,” and represents the TH sound. As Old English developed into Middle English, this “-aþ” suffix was lost.
The rest of the syncretism in present-tense forms came later. The next piece to fall into place was the first-person singular form. When the plural “-aþ” suffix was lost, the plural form ended up sounding almost indistinguishable from the first-person singular, which was “deme” [day-muh]. That just leaves the second-person singular form, “demest” [day-mest].
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