A hyphen is used to join words or parts of words.  They are often used in compound modifiers when the modifier comes before the word it’s modifying.

A compound modifier is made up of two or more words that work together to function like one adjective in describing a noun. When you connect words with a hyphen, you make it clear to readers that the words work together as a unit of meaning.

• She sells one-of-a-kind jewelry in her online store.
• We're looking for a kid-friendly restaurant.
• My new car has state-of-the-art features.

When combining an adjective, a noun, or an adverb that doesn’t end in -ly with a present participle (the –ing form of a verb) to describe another word, use a hyphen to make the meaning of the combined descriptor clear.

• I only use fast-acting medication when I have a headache.

Don’t use a hyphen when the modifier comes after the noun it’s describing.

• This medication is fast acting.

Don’t use a hyphen when you are combining an adverb ending in -ly and a participle.

• INCORRECT: I prefer simply-made meals.
• CORRECT: I prefer simply made meals.

When using high or low as part of a compound modifier, use a hyphen when the compound comes before the noun it’s modifying.

• I use low-cost ingredients in my stew recipes.
• My high-efficiency washing machine uses less water than older models. 

Hyphenated compound words are multiword terms with a hyphen or hyphens between their component words. Over time, many hyphenated compounds become closed compounds.  For example, teen-ager became teenager. Check a dictionary if you’re not sure how to treat a compound.

• Father-in-law
• Ten-year-old
• President-elect

Numbers between twenty-one and ninety-nine should be hyphenated when they’re spelled out.

• I've missed twenty-three classes so far this year.
• My assignment is due in forty-eight hours.

Spelled-out two-word simple fractions are also hyphenated. 

• The recipe calls for one-third cup of brown sugar.

One-word simple fractions (such as half and quarter) as part of a compound are not hyphenated.

• Tim waited a half hour for Grace to call.

When a number appears as the first part of a compound modifier that comes before a noun in a sentence, the compound modifier is hyphenated. This applies whether the number is spelled out or in numerals, and whether it is cardinal or ordinal.

• The instructor gave a thirty-minute speech on the importance of soft skills.
• There is a 88-foot-tall sign in front of the stadium.
• I'm taking a class on fifteenth-century pottery.

A hyphen is not required if the number is the second word in the compound adjective.

• He has type 2 diabetes.
• I need to know how to get to the basement 3 lab.