Learn about how hyphens modify a phrase. In a three-word phrase, the hyphen is placed between the first two words and joins those words together. The first word, which is a noun or adjective, is modifying the second word, which is also a noun or adjective. However the first two words are not modifying the third word, which is a noun. For example, "third-grade teacher" refers to a teacher who teaches third grade. It does not refer to someone's third teacher. As another example, a "one-way valve" refers to a valve that only allows flow in one direction. It does not mean one valve.
Look for when two or more adjectives are placed before a noun in a sentence.
Red-hot coals, for instance, describes just how hot the coals are: red hot. Red, hot coals would just be coals that happen to be red and hot. The hyphen lets the reader know that red describes (modifies) hot, and together the hyphenated word describes coals.
Use a hyphen when adverbs, not ending in "ly" are placed in front of a noun. For example, a well-known scholar, or a long-awaited movie. Conversely, never use a hyphen when an adverb ending in "ly" is modifying an adjective (which is modifying a noun). For example, "the swiftly moving river" is correct. "The swiftly-moving river" would be incorrect. The reason for this is that "swiftly" clearly does not modify "river," so the reader understands that it automatically must modify "moving."
Place a hyphen between all compound numbers through ninety-nine. This includes writing fractions. For example, if someone is celebrating their forty-first wedding anniversary, a hyphen is placed between the words. Another example is when fractions are used in a written sentence. For example, two-thirds of the population voted in the previous election.
Place a hyphen where words have become interlinked because of the current vernacular. These you might have to look up in the dictionary for clarification as there is no exact rule. For example, terms such as father-in-law, mother-in-law and even in-laws have all evolved over time to be written with a hyphen. Other shortened terms such as X-ray have become part of both the spoken and written language.
Hyphenate prefixes that occur before a proper noun. For example, if a person or group is anti-Semitic or un-American.
Use a hyphen when writing words with the prefix "self" except for selfish and selfless. For example, self-started, self-motivated and self-worth would all be hyphenated.
Check to see if the prefix ends with the same letter that begins the root word. This is one of those tricky English language rules. Examples include anti-immigration or semi-interested. Notice both the last letter of the prefix and first letter of the word are "i."
Place a hyphen after the prefix "ex." Examples include ex-husband, ex-wife, or ex-fiance among others.
Refer to the dictionary or credible dictionary website. Because there often is no rule on when to and when not to use a hyphen, it might just be easier to look it up to ensure that you are correct.