Compound nouns

A compound noun is a noun that consists of two or more different words. Most compound nouns are formed by combining two nouns or an adjective and a noun. Toothpaste (noun+noun) is an example of a compound that combines two nouns. Greenhouse (adjective+noun) is a combination of an adjective and a noun.

Compound nouns can also be formed in the following ways 

Noun	+	Noun	bodyguard
			travel agent
			chat-room
Adjective	+	Noun	greenhouse
			full moon
			highway
Preposition	+	Noun	underground
			input
			bycatch
Verb	+	Noun	driving license
			answering machine
			breakfast
Noun	+	Preposition	hanger on
			love-in
Noun	+	Verb	haircut
			rainfall
Verb	+	Preposition	check-in
			build-up
Adjective	+	Verb	dry cleaning

Compound nouns are written as a single word or separate words. Hypens are also used in some compound nouns.

1. closed or solid compound nouns (one word): snowman, sunflower, bathroom, etc.
2. open or spaced compound nouns (two separate words): car park, washing machine, etc.
3. hyphenated compound nouns (joined by a hyphen): mather-in-law, fire-fly, check-out, etc.

Plurals of compound nouns

Closed compound nouns form their plurals by adding  -s/-es to the end.

Singular	Plural
whiteboard	whiteboards
toothbrush	toothbrushes
haircut	haircuts

Compound nouns written with hyphens or spaces pluralize the base word.

Singular	Plural
attorney-general	attorneys-general
father-in-law	fathers-in-law
heir apparent	heirs apparent
man-of-war	men-of-war
sergeant major	sergeants major
step-parent	step-parents
washing machine	washing machines

Related topics:

Collective nouns

Plural nouns

Common nouns

Proper nouns

Gender-specific nouns

Verbal nouns