"Now I, Nephi, do speak somewhat concerning the words which I have written, which have been spoken by the mouth of Isaiah. For behold, Isaiah spake many things which were hard for many of my people to understand; for they know not concerning the manner of prophesying among the Jews." (2 Nephi 25:1).
What did Nephi mean when he said that Isaiah spake things that were hard for his people to understand? The obvious answer to that question is elucidated by the last part of the verse: "for they [knew] not concerning the manner of prophesying among the Jews." His people did not generally have the education or experience or knowledge to fully understand Isaiah's prophecies. Maybe they didn't have the desire to understand Isaiah. I think this holds true for many people today. The common 'complaint' I hear about Isaiah is that the book is difficult to understand. It can be if you do not understand that Isaiah's language is highly symbolic but at the same time it is very direct and literal, meaning it refers to specific events. It's also repetitive; meaning that Isaiah states the same thing multiple times in multiple ways.
I will not get into ways of understanding Isaiah - the best way is to spend time reading the book and praying for understanding - because that is not the purpose of this post. I wanted to share another quote and insight from Hugh Nibley about what he thinks is the reason that Isaiah is "hard...to understand". I have been quoting him a lot recently because I have been reading his Teachings of the Book of Mormon Part 1, which is a transcript of lectures he gave in his Book of Mormon class at Brigham Young University in the 1980s.
"'Isaiah spake many things which were hard for many of my people to understand.' Isaiah himself often mentions the fact that the people ask him to speak smooth things [i.e., the people want Isaiah to tell them only things that make them feel good and that they want to hear]. They want to hear smooth things. I am not going to teach you smooth things, he says. If I just gave you the smooth things you want, you wouldn't need them.... If the scriptures told us only what we wanted to hear, of course we wouldn't need them.
"You notice it all changed under the rabbis; the interpretations became different. Isaiah is much too literal [for them], etc. Then, of course, they accepted the University abstractions and became more philosophical and intellectual in the interpretation of everything." (Nibley, Teachings of the Book of Mormon Part 1, 2004, p.249).
In short, Hugh Nibley's explanation of why Isaiah is hard to understand is because Isaiah does not speak "smooth" things to the people, he told them things that were hard to hear because he condemned their wickedness. He was a prophet who simply "told it like it was." I think Hugh Nibley's insight adds to our understanding of Isaiah. Yes, his writings can be difficult to understand because of his language and his symbolism, but then again, the book of Isaiah is not as difficult to understand as many people believe. Isaiah simply wrote many difficult things for the wicked to understand, for he prophesied about the Messiah and the temple and the way to salvation. He wrote of many things that would transpire in the last days - in our day - the restoration of the gospel, the building of temples, and so forth. Great are the words of Isaiah.