Compounds can confuse some people, especially since compound words may represent different parts of speech. Typically, you will notice when this occurs, this kind of compound may change -- such as an open compound to a closed compound. For example, the word "carryover" is a closed compound and can denote an adjective or noun. However, the words "carry over" is an open compound and depicts a verb.
Most compound words are in the dictionary. However, you may have an issue when the compound word does not appear in the dictionary. This normally occurs when someone creates a compound word for a certain circumstance; and it is not readily used. When this occurs, you can check the Associate Press Style Manual or the Chicago Manual of Style for guidance.
Some examples of closed compound words include: keyboard, firefly, dragonfly, softball, redhead, makeup, secondhand, notebook, breakfast and oatmeal.
Examples of some open compound words include real estate, full moon, half sister, attorney general and post office.
Words considered as hyphenated compounds include over-the-counter, eight-year-old, mass-produced, son-in-law and master-at-arms.
Closed compound words are typically the easiest type of compound word to make plural. In the English language, you generally create plural compound words, just like regular words, by adding the letters "s" or "es" to the base word. Some examples of plural closed compound words include headmasters and toothbrushes.