Is there a list of nouns and their plurals?
E.g. apple apples, genius geni, roof rooves, etc.
I have the GCIDE dictionary but that only contains singular nouns.
This python package can pluralize words for you, you can just run your entire dictionary through it to get the plural forms: http://www.clips.ua.ac.be/pages/pattern-en#pluralization
For this you first need Python, I myself use winpython, but any python installation would do. You can then download the pattern package here: http://www.clips.ua.ac.be/pattern