Moroni tells us that the plates are not written in Hebrew because that would have taken more room on the plates. Hebrew is a condensed writing system in that it leaves out the vowels of the spoken word. While this is typically no great problem to reconstruct, there are times when there are some ambiguities in a word that might be read with different vowels. Thus Hebrew writing inherently has issues of potential confusion on the intent of a certain word. Moroni is telling us that in some way, the writing system that they are using is more prone to error than the Hebrew. Even with the possibilities of error in supplying the vowels, that mode of writing would have been more accurate than the method that was used. Moroni links as least some of the imperfections in his father’s work to the difficulty in accurately communicating through their writing system.
Next, the very fact that Moroni suggests that they could have written in Hebrew and did not, tells us that it was preserved as a scholarly language. There must have been some writings in Hebrew from the earliest days, or Moroni would not have been able to have learned Hebrew to know that it could have been used.
Lastly, the statement about Hebrew must be combined with the information from the previous verse. In that verse, Moroni tells us what they used instead of Hebrew: “the characters which are called among us the reformed Egyptian, being handed down and altered by us, according to our manner of speech.” (italics added). The language us called reformed Egyptian. We are not told what relationship the reformed Egyptian has to the Egyptian that is in Nephi’s record, but it Moroni is telling us that they are not the same. Something has happened that has changed the form, allowing Moroni to note that it is reformed.
The next important aspect of this description is that there is a relationship between the characters and their “manner of speech.” These characters have a relationship to speaking, and it appears to be a relationship that is not enjoyed by the Hebrew that was not used. Moroni appears to be telling us that that the reformed Egyptian is a representation of their spoken language. While he does not clearly state that Hebrew is not, there is a rather strong implication that we should not seek Hebrew meanings underneath the words we see. Their language would have been best represented by the reformed Egyptian.