We add a prefix to a word to make a solid word unless there is already a word with that spelling and a different meaning. This occurs most often with the prefix “re-.”
…She promises to work to reform her bad behavior.
…After the merger that occurred, we had to re-form the company.
…I have decided to resign from the board.
…We returned to the bank to re-sign the papers.
A couple of interesting examples along these same lines:
…The books on the shelf seem to just multiply over the months.
…It was a multi-ply board.
…The people unionized in an effort to improve working conditions.
…It was an un-ionized compound.