James B. Allen answers:

A Professor Joshua Seixas was hired by Joseph Smith and his associates to teach them Hebrew. They originally hired another teacher who turned out to be very unsatisfactory, but they were determined to learn the language as well as they could. So, On November 21, 1835, they agreed to send someone to New York to find a Jew who was more qualified to teach them. On January 4, 1836, however, William E. M'Lellin and Orson Hyde were dispatched to the Hudson, Ohio, to find the right person. When M'Lellin returned on January 6, 1836, he reported that he had hired a teacher who was "highly celebrated as a Hebrew scholar, and proposes to give us a sufficient knowledge during the above term to start us reading and translating the language." (Joseph Smith, History of the Church, 2:356). It was going to cost $320 for seven weeks, to have him teach forty "scholars." He would arrive in about 15 days. Meanwhile, the group continued to study as best they could. On February 15, according to Joseph Smith's History, they began "translating the Hebrew language, under the instruction of Professor Seixas, and he stated that we were the most forward of any class he ever instructed for the same length of time." (2:396). It was this same Professor Seixas who published the Hebrew grammar they used. Seixas was apparently a professor at Oberlin College.

In 1834 Joshua Seixas published a little book titled Manual. Hebrew Grammar For the Use of Beginners, by J. Sexias, 2d ed. enlarged and improved. This was the manual that was used in the Kirtland school when Seixas came to teach Hebrew to Joseph Smith and his associates in 1836. In 1981 this manual was published in a facsimile edition by the Sunstone Foundation, Salt Lake City. It carried an introduction by Louis Zucker, entitled "Joseph Smith As a Student of Hebrew." This introduction had previously been published, under the same title, in Dialogue: A Journal of Mormon Thought 3 (Summer 1968): 42-55. Zucker, not a member of the Church, had been a respected member of the University of Utah faculty since 1928. The following paragraph is taken from page xiv of that manual.

"In Joseph's use of Hebrew outside of the Mormon Scriptures, we find a tiny, little sentence, like those in Seixas's Manual (1834, pp. 87 ff.) but simple–Ahtau ail rauey, Thou O God seest [me]–and the name "Nauvoo." Now, In April, 1839, Joseph Smith, surveying from a hill the wild prospect around Commerce, imagining what he could do with it, thought, "It is a beautiful site, and it shall be called Nauvoo, which means in Hebrew a beautiful plantation." B. H. Roberts comments: "The word Nauvoo comes from the Hebrew, and signified beautiful location: ‘carrying with it also,' says Joseph Smith, ‘the idea of rest.'" Many have scoffed at the assertion that the name is Hebrew, but it is. In Seixas's Manual (1834, p. 111), in a List of Peculiar and Anomalous Forms Found in the Hebrew Bible, the first words under the letter Nun are na-avauh and nauvoo–verb forms whose anomalous "voice" is designated, without translation. The first word the Authorized Version renders "becometh" (Psalms 93:5) and the word nauvoo is rendered "are beautiful" (Isaiah 52:7), "are comely" (Song of Solomon 1:10). This verb may be used of person, thing, or place. The idea of rest may have stolen in from idyllic verse two of the Twenty-Third Psalm, where a homonymous root is used meaning "pastures" (ne-ot or ne-oth)."