Why would Joseph Smith say that Moroni waved the “rent” of his garment?

In the original 1830 version of the Book of Mormon it says in Alma 46:

And when Moroni had said these words, he went forth among the people, waving the rent of his garment in the air, that all might see the writing which he had wrote upon the rent, and crying with a loud voice… (emphasis added)

John A. Tvedtnes explains:

When the word “rent” is used as a noun in English, it may refer to a hole caused by rending, but not, to my knowledge, to a portion of rent cloth; the unlikely usage of “rent” in English as a noun no doubt contributed to the fact that, in subsequent editions of the Book of Mormon, it was changed to read “rent part” (Alma 46:19). But the Hebrew would, in this instance, use but one word, qeraʿ, “rent (part),” coming from qāraʿ, “he rent, tore,” for nouns, in Hebrew, are derived from roots—as are Hebrew verbs—by the addition of certain vowel patterns that distinguish them from other parts of speech.
– John A. Tvedtnes – Hebraisms in the Book of Mormon: A Preliminary Survey

Where would Joseph Smith have learned so much Hebrew? Would he really have added this in to make the book sound more authentic?

See:

John A. Tvedtnes – Hebraisms in the Book of Mormon: A Preliminary Survey

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