As you know, a noun is a person, place or thing, and an adjective is a word that describes a noun:

adjectivenoun
cleverteacher
smalloffice
blackhorse

Sometimes we use a noun to describe another noun. In that case, the first noun "acts as" an adjective.

noun as
adjective
noun
historyteacher
ticketoffice
racehorse

The "noun as adjective" always comes first

If you remember this, it will help you to understand what is being talked about:

  • a¬† race horse¬†is a¬†horse ¬†that runs in races
  • a¬† horse race¬†is a¬†race ¬†for horses
  • a¬† boat race¬†is a¬†race ¬†for boats
  • a¬† love story¬†is a¬†story ¬†about love
  • a¬† war story¬†is a¬†story ¬†about war
  • a¬† tennis ball¬†is a¬†ball ¬†for playing tennis
  • tennis shoes¬†are¬†shoes ¬†for playing tennis
  • a¬† computer exhibition¬†is an¬†exhibition ¬†of computers
  • a¬† bicycle shop¬†is a¬†shop ¬†that sells bicycles

The "noun as adjective" is singular

Just like a real adjective , the "noun as adjective" is invariable. It is usually in the singular form.

RightWrong
boat raceboat racesNOT  boats race, boats races
toothbrushtoothbrushesNOT  teethbrush, teethbrushes
shoe-laceshoe-lacesNOT  shoes-lace, shoes-laces
cigarette packetcigarette packetsNOT  cigarettes packet, cigarettes packets

In other words, if there is a plural it is on the real noun only.

A few nouns look plural but we usually treat them as singular (for example news, billiards, athletics). When we use these nouns "as adjectives" they are unchanged:

  • a news reporter, three news reporters
  • one billiards table, four billiards tables
  • an athletics trainer, fifty athletics trainers

Exceptions:  When we use certain nouns "as adjectives" (clothes, sports, customs, accounts, arms), we use them in the plural form:

  • clothes shop, clothes shops
  • sports club, sports clubs
  • customs duty, customs duties
  • accounts department, accounts departments
  • arms production

How do we write the "noun as adjective"?

We write the "noun as adjective" and the real noun in several different ways:

  • two separate words (car door)
  • two hyphenated words (book-case)
  • one word (bathroom)

There are no easy rules for this. We even write some combinations in two or all three different ways: (head master, head-master, headmaster)

How do we say the "noun as adjective"?

For pronunciation, we usually stress the first word:

  • shoe¬†shop
  • boat-race
  • bathroom

Can we have more than one "noun as adjective"?

Yes. Just like adjectives, we often use more than one "noun as adjective" together. Look at these examples:

car production costs: we are talking about the costs of producing cars

noun as
adjective
noun as
adjective
noun
  costs
 productioncosts
carproductioncosts

England football team coach: we are talking about the coach who trains the team that plays football for England

noun as
adjective
noun as
adjective
noun as
adjective
noun
   coach
  teamcoach
 footballteamcoach
Englandfootballteamcoach
Englandfootballteamcoaches

Note: in England football team coach can you see a "hidden" "noun as adjective"? Look at the word "football" (foot-ball). These two nouns (foot+ball) have developed into a single noun (football). This is one way that words evolve. Many word combinations that use a "noun as adjective" are regarded as nouns in their own right, with their own dictionary definition. But not all dictionaries agree with each other. For example, some dictionaries list "tennis ball" as a noun and other dictionaries do not.

government road accident research centre: we are talking about a centre that researches into accidents on the road for the government

noun as
adjective
noun as
adjective
noun as
adjective
noun as
adjective
noun
    centre
   researchcentre
  accidentresearchcentre
 roadaccidentresearchcentre
governmentroadaccidentresearchcentre

Newspapers often use many nouns together in headlines to save space. Look at this example:

BIRD HEALTH RESEARCH CENTRE MURDER MYSTERY

To understand headlines like these, try reading them backwards. The above headline is about a MYSTERY concerning a MURDER in a CENTRE for RESEARCH into the HEALTH of BIRDS.

Note, too, that we can still use a real adjective  to qualify a "noun as adjective" structure:
  • empty ¬†coffee jar
  • honest ¬†car salesman
  • delicious ¬†dog food
  • rising ¬†car production costs
  • famous ¬†England football team coach

sources : Original Link