The ten hundred most common words
PowerShell v3+, 105 92 bytes

param($a,$b)$x=@();-split($b-replace"[^a-zA-Z']",' ')|%{if($_-notin$a){$x+=$_}};($x,1)[!$x]


Takes simple words like $a, and words like $b. Makes helper $x. Take each word in $b and get rid of any bad not letters, then check each one |{...}. If that word is not in $a, then we add it to $x. At the end, we choose $x or 1 by not $x. That is sent out, either words or 1.

Some words to try

PS C:\Tools\Scripts\golfing&gt; ('This returns "Hello, World!"','tHiS rEtUrNs TrUe...','Thing Explainer is a book written by a man.
The man writes books with simple words.','This set of stuff "¤!^¤&gt;7\ must return true'|%{"$_";(.\ten-hundred-most-common-words.ps1 (gc .\ten-hundred-most-common-words.txt) $_)})-join"`n###`n"
This returns "Hello, World!"
###
1
###
tHiS rEtUrNs TrUe...
###
1
###
Thing Explainer is a book written by a man.
The man writes books with simple words.
###
1
###
This set of stuff "¤!^¤&gt;7\ must return true
###
1

PS C:\Tools\Scripts\golfing&gt; ("This code doesn't returns Hello, World!",'tHiS rEtUrN"s false...'|%{"$_`n***`n"+(.\ten-hundred-most-common-words.ps1 (gc .\ten-hundred-most-common-words.txt) $_)})-join"`n###`n"
This code doesn't returns Hello, World!
***
code
###
tHiS rEtUrN"s false...
***
s false