What is it called when you coin a different version of a word that already exists?
Solution 1:
The process of doing this, and the result, is called back formation.
You take a perfectly good word, and not knowing the form you want, you form a new word using established rules, which happen to be wrong in the case in point.
An example from abolish. The proper noun here is abolition, but perhaps on the pattern of forms like astonishment or punishment, the back-formation abolishment is often heard. To me, this is a cause for astonition and the people who do it should suffer punition.