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.*
800 years is a LOT of language evolution, but if they kept the alphabet, numbers and phonetics remained the same it would be a LOT easier than an unknown language in an unknown script.

A newspaper is written for a 10 year olds reading age, childrens English primers might be there as well. Textbooks, a whole bunch of stuff that would help.

The roots of words should still be discernible, Boewulf was written much longer than 800 years ago in a different script in Old English, but the Middle English version can be made out better with a bit of guesswork mainly because it's in our script

Your best bet is English primers and suchlike to get a feel for the language and basic vocab. Having said that English is one of the most complicated languages, so hopefully they have retained the grammar structure which is plausible since we have basically retained that since before English became English with no real changes.