Hyphens are a notoriously tricky punctuation mark. English is a fairly inconsistent language and the student is not helped by the constant trend of evolution of the rules. To ensure absolute correctness when using hyphens, keep a good dictionary nearby, for knowing when to hyphenate compound words is not a definite science.
Compound words are two or more individual words that have a different or enhanced meaning when combined. For instance, “bumble” can mean to drop or fumble something, “bee” can be either an insect or a gathering, but a “bumblebee” is a distinct word used for a specific flying insect. Compound words are either written as one word, two words, or two words connected by a hyphen. Therein lies the complications, as no one seems able to properly define when to hyphenate compound words at all.
A common rule in determining whether a compound word should be one word or two is how the word is used in context. Phrases that have a verb/noun or verb/adjective combination should be two words if the combined term is a verb and one word if the compound word is a noun or description. For instance, “my car will never break down” uses “break” as a verb, whereas “Michael had a breakdown” uses “break” as part of a noun. This is useful in determining the difference between one and two word compound phrases, but does not really help us with whether to hyphenate compound words or not.
One usually consistently hyphenated form of compound words are those with a connecting middle word. Terms such as merry-go-round or mother-in-law are almost always written with a hyphen. One exception is commander in chief, which is frequently seen without the hyphen, especially when referring to the President of the United States as an alternate title.
Knowing the linguistic origin of may help determine when to hyphenate compound words. Many compound words with a Germanic root are written as one word, such as fisherman. Still, as with all hyphenating words, this is not entirely consistent, and having at hand knowledge of the early origin of most words is a skill not many possess.
The best guideline to remember when dealing with common words is that extremely common terms, like someday, always, and highway, are rarely hyphenated. If you are using a new or unusual compound, check in a dictionary to see if the term exists, and if so, how it is punctuated. If you cannot find the word, it is best to hyphenate it, as it is clearly not in common usage.
Don’t panic when worrying about whether to hyphenate compound words. The rules are so uncertain that it is rare to be docked grammatical points on essays or tests for incorrect hyphenation. If you are accused of misusing or neglecting the hyphen, go to your professor or the person who graded your work and politely ask them to show you some proof that your usage is incorrect. Chances are, they will be guessing, too!