Two or more words can be combined to form what is called a compound word. This is particularly true of adjectives and nouns.
For example, the words play and house can be combined to form the compound word playhouse. Unfortunately, there is no way to know how to correctly combine all compound words other than consulting a dictionary (or, of course, through memorization).
There are three different forms of compound words: open form, hyphenated form, and closed form.
Open form—With this form of compound word, each word is written separately; for example, school bus.
Hyphenated form—With this form, the words forming the compound word are separated by a hyphen; for example, merry-go-round.
Closed form—With this form of compound word, the two words are written as one; for example, stingray.
When it comes time to take the exam, you will not be permitted to take a dictionary in with you. Therefore, you will have to rely on your own knowledge and skill to determine whether a hyphen is required. Here are a few points to keep in mind that will help you do this:
Determine whether the meaning of the sentence might change if a hyphen is not used.
If a word is added to a past participle that precedes a noun, a hyphen is required; for example, well-respected professor.
Hyphens are not required if the compound word is created using ly.