Q. What is a hyphen?
A. Hyphens are used to divide or to compound words, word elements, or numbers
Q. When do I use a hyphen to combine two words?
A. Use a hyphen to combine two words when modifying a noun
Q. Do I use a hyphen before a prefix?
A. Generally not, although the rule varies from word to word. When in doubt, visit m-w.com and look up the prefix (pre-, under-, etc.). Often you can find the correct usage of the word you are looking for in the word list under the prefix entry
Q. What’s the difference between a hyphen and a dash?
A. There are three types of dashes:
Final fact: One never uses spaces on either sides of a dash.