Keep in mind that a compound noun is any sequence of two or more words used as a single nominative expression. For example, "jet pilot," "daughter-in-law" and "firecracker" are all compound nouns.
Remember that there are three ways of writing compound nouns: closed forms, such as "firefly" and "softball"; open forms such as "pearl diver" and "post office"; and hyphenated forms such as "pea-soup" and "daughter-in-law."
Note that over time, open-form compound nouns tend to morph into closed forms after many years of usage. According to an online grammar resource maintained by City Colleges of Chicago, a closed-form compound noun like "firefly" originated as two separate words ("fire fly") that were later hyphenated ("fire-fly") before finally being joined, as in common in contemporary usage at the time of publication.
Remember that compound nouns behave grammatically the same way as non-compound nouns in a sentence. This means you can use a compound noun "office building" just as you use the word "building." Both forms are considered singular for the purpose of verb agreement.
Use a hyphen with a modifying compound to avoid uncertainty over which word the compound is modifying. For example, write "the old-music lover" when you are describing someone of any age who loves old music; write "the old music lover" if you are describing a music lover who is himself old.
Form the plural of compound nouns usually by adding "s" to the main, or base, word of the compound noun. The main word is generally the most significant word. For example, write "passers-by," "fathers-in-law," "deputy sheriffs" and "half-moons." Look up the compound noun in an authoritative dictionary like "Merriam-Webster's Collegiate Dictionary" if you're uncertain of the word's plural form.
Use only the part of the compound noun preceding the hyphen when you are listing a series of hyphenated compound nouns. For example, write "The grandmother was surrounded by her many sons- and daughters-in-law."
Write the possessive form of a singular compound noun in the usual way by adding an apostrophe and an "s" to the end of the noun. For example, "The half-moon's sheen was lovely to see."; "The police sergeant's dog bit my leg."
Avoid a string of "of's" when using the possessive forms of plural compound nouns. For example, instead of the awkward "The collections of wrenches of master mechanics are wondrous," write "The wrench collections of master mechanics are wondrous."