The rule is that a prefix is added to the front of a word to make a solid word.
...preordained
...postprandial
...overrated
...underfed
However, when there is already a word that has a different meaning, the prefix should be hyphenated. This usually occurs with the prefix re-.
...He decided the only choice was to resign.
...We returned to the bank to re-sign the papers.
...He was looking for redress for the grievance.
...Her mother demanded that she re-dress for the more formal occasion.
And here is a really fun one.
...The decision was made that the company should be unionized.
...It was an un-ionized element.
Happy punctuating!
Margie