Is it simple or is it hard?
R, 106 bytes

Is not sure if is understand challenge because is has hard time reading.



function(s){u=toupper;all(strsplit(gsub("[^A-Z -']","",u(s)),"[ -]")[[1]]%in%u(scan("most used.txt","")))}


This creates an unnamed part of a computer thing that accepts a string and returns something like a true or like a not true.

Ungolfed + explanation:

partOfAComputerThing &lt;- function(s) {
    # Remove everything but letters, spaces, dashes, and single quotes
    s &lt;- gsub("[^A-Z -']", "", toupper(s))

    # Split s into a vector on spaces/dashes
    v &lt;- strsplit(s, "[ -]")[[1]]

    # Read the file of words (assumed to reside in the current directory)
    m &lt;- scan("most used.txt", "")

    # Determine if all words in the input are in the file
    all(v %in% toupper(m))
}


Thanks to Dennis for inspiration thing.