A compound noun is a noun made from at least two words. There are three forms for compound nouns: (1) with spaces, (2) without spaces, and (3) with hyphens.

Compound nouns often have a meaning that is different, or more specific, than the two separate words. You have noticed that the compound noun can be written either as a single word, as a word with a hyphen, or as two words. There are no clear rules about this.

Compound nouns are sometimes one word, like toothpaste, haircut, or bedroom. Sometimes compound nouns appear as two separate words: full moon, Christmas tree, and swimming pool are some examples of compound nouns that are formed with two separate words. These are often referred to as open or spaced compound nouns.

When you use three words together as a single thought describing or modifying a noun and you put them before the noun, you should hyphenate them: It was a matter-of-fact decision.

First we’ll talk about when you do hyphenate an age: You do it when the age is acting like a noun and when the age is an adjective that comes before the noun and modifies the noun. In this example, the age—70-year-old—is used as a noun, and you hyphenate it: That 70-year-old with a purple hoodie loves Justin Bieber.

The hyphens are unnecessary in sentence 1 because the phrase “five years of age” is not being used as a single adjective. The hyphen in sentence 3 is correct because we always use a hyphen in compound numbers between twenty-one and ninety-nine.

In the expression “5 year old,” the word ”year” functions as an adjective and we do not use the plural form of the word. In the other expression, “5 years old,” the word “year” is a noun and we thus use the plural form. (He is five years old.)

Follow these rules for writing ages: Write out ages one to nine. Use numerals for ages 10 and older. Ages noted in years and months should be written in numerals.

Examples of hyphenated compound words include:

• two-fold.
• check-in.
• merry-go-round.
• father-in-law.
• seventy-two.
• long-term.
• up-to-date.
• mother-in-law.

Compound words can be written in three ways: as open compounds (spelled as two words, e.g., ice cream), closed compounds (joined to form a single word, e.g., doorknob), or hyphenated compounds (two words joined by a hyphen, e.g., long-term). Sometimes, more than two words can form a compound (e.g., mother-in-law).

Examples of Compound Words

Open Compound Words Examples

• High school.
• Living room.
• Roller coaster.
• Peanut butter.
• First aid.
• Web page.

In English grammar, closed class refers to the category of function words—that is, parts of speech (or word classes)—that don’t readily accept new members. In contrast open class words include nouns, lexical verbs, adjectives, and adverbs.

A simple sentence consists of only one clause. A compound sentence consists of two or more independent clauses. A complex sentence has at least one independent clause plus at least one dependent clause. A set of words with no independent clause may be an incomplete sentence, also called a sentence fragment.

Three methods of forming compound sentences

• with a coordinating conjunction (one of the fanboys);
• with a semicolon; or.
• with a semicolon and a transitional expression.