If you often wonder whether you need to hyphenate a compound word, take heart in knowing that hyphens can be annoyingly tricky, even for the best writers and editors. The Chicago Manual of Style advises that you first look up the compound word in a dictionary. If you can't find it, try to figure out the grammatical function of the word. Is it a noun, an adverb, or an adjective that modifies a noun? If so, there's a pretty good chance it's hyphenated.
Here are some examples: a four-year-old ate the cookie; the blue-eyed girl smiled; a full-time student at the university. You can also reference this helpful hyphenation guide from the Chicago Manual of Style. Or, you can always jump on YouTube and check out this Schoolhouse Rock video.