Hyphens are typically used as connectors; that is, hyphens indicate a relationship between two words.   Hyphens are often used with adjectival phrases; that is, if a single word does not exist to properly modify a noun, use a hyphen to join two words.   These two words function as a single unit and modify a noun together.  Without a hyphen, the phrase may confuse the reader as to what exactly your adjective is modifying.

Consider the following examples, both of which are legitimate sentences; however, the meanings are dramatically different.  

1.         The small car accident happened on the corner of Elmhurst Drive and Regal Boulevard.

2.         The small-car accident happened on the corner of Elmhurst Drive and Regal Boulevard.

Notice the only difference between the two sentences is the hyphen.  What exactly does the word "small" modify?   The size of the accident?  The size of the cars in the accident?  In the first sentence "small" is modifying "car accident" (the car accident did not involve many cars) while in the second sentence only "small cars" were involved in the accident.

Not all uses of hyphens can be confused as above.  Consider these examples:

1.         The student-run organization grew from 3 members to 30 members in one year.

            (Remember, "run" by itself does not modify organization – you need both words)

2.         The CEO's inappropriate comment destroyed a long-standing relationship with a high-profile client.

PLEASE NOTE: You do NOT need a hyphen if one of the words is an adverb (usually ends in "ly").

EXAMPLE:

1.         The neighbors reported a highly suspicious person to the police two weeks ago.

2.         Overly protective parents can stifle the social development of their children.