Very often the use of a hyphen is a staging post towards the merging of two words into one.
An example is wordprocessor. Many of us now accept this written as a single word or hyphenated as in word-processor. Other words that were once hyphenated but are now accepted as complete words include common nouns like weekend and bypass. Your best bet with hyphenation is to choose one good dictionary and follow that single source to determine whether or not to hyphenate a word. However, having said all that, there are a few other guidelines worth following …
For a start, when you are dealing with a compound adjective consisting of two words, the second of which is a participle or an -ed word, you will usually use a hyphen.
This practice results in words like good-natured, and best-selling (although this is rapidly becoming bestselling) and never-ending. Also, when using the terms half, full, or part, it is usual to use a hyphen, as in: part-time, and full-time, and half-time. The same applies when using words that express a degree of quality (better, worse, well, ill, fast, slow and so forth) as we see in well-known, ill-suited, and also best-loved. Perhaps the most sensible use of the hyphen is to clarify meaning.
A hyphenated quick-reference book is preferable to a quick reference book, when we want to indicate the book’s main use is for quick reference. On the other hand, you’ll find hyphens are fast disappearing (in other words, these ones are fast-disappearing hyphens …) in usage when words have a re- or co- prefix. We now tend to have rewrite, reread, reset, rethink and similar words without hyphens.
However, where the syllable following the prefix begins with e, we normally retain the hyphen as a pronunciation aid as in: re-entry, re-evaluate, re-establish.
Oddly enough, we tend not to use a hyphen in co- words, even where the second syllable starts with an o. So we would usually have: cooperate and coordinate and coauthor (although we still have co-opt and co-own).