Question: Out in the former state of Utah, near where the Old Salt Lake ruins are, a group of scholars and students from the New Jerusalem University are on an exploration mission. They have heard, from an unknown but (maybe) reliable source that an old Fallout Shelter is out there. They search all day until they find it. They ask, or rather force, some slaves to break down the door, and they are able to get in. 

The Fallout Shelter, though littered with skeletons, is full of valuable knowledge from before WWIII. The paper they used in the Old Times was super powerful, and never rotted, at all. Most of the notes the found were just grocery lists and etc. but they do find a blueprint which talks about some machine powered by that mystical force, the “electron” if I remember correctly. 

There is just one problem. The apocalypse was 800 years ago, and since that time New English (The language they speak in the present) is totally different from Modern English. So, what might be a plausible reason for why the college kids could read Modern English?

Answer: 
Languages tend to develop fastest in cities, where lots of people meet and develop new words and new grammar. Outside the cities, especially in isolated communities, language change is slower. There are parts of Sardinia that have an Italian dialect that would probably be intelligible to the Romans. If this is the case it may be that a slave can read what the intellectuals cannot.

*Modern English is a foreign language to the intellectuals of New Jerusalem, but out in the wilds of Utah, among the backwoodsmen, you will still find families who speak a language that, while it isn't classical English, could be understood by a person from the times before the war.* 

*Among the slaves is LeVerl, and he grew up in an old isolated family (before his capture and enslavement), and while he has since learned to speak New, he still knows the language that his mother learned from her father. To the scholars’ surprise he can read the old texts.*
In school, some of us learn Latin. We learn it because it is the root of many modern languages, so learning Latin helps us with understanding the modern languages.

Classical Latin is 2000 years old. Also, the fact that they call their language New English suggests that there's strong connections between New English and "modern" English (at least similar as "modern" English to Old English, though that's not very similar).