Noun adjunct

  Noun Adjuncts (noun + noun)

          In English we can put two nouns together. The first noun is used as an adjective to modify the second noun and is called a noun adjunct. The first noun is almost always singular because it follows the rule for adjectives, which do not have plural forms in English.
 N₁ N₂ can mean that
a. N₁ is a kind of N₂ (a grammar book is a kind of book)
b. N₁ is an object of an implied verb (an apple tree is a tree that produces apples)
       It is important to understand that N₂ is the thing and N₁ is the kind or type:
a rose bush is a busha wrist watch is a watchcomputer paper is paper
N₁ is singular, even if the phrase is plural.
Examples:

My mother planted a rose bush in the garden.
I bought a new table lamp.
Bobby takes the school bus to school.
The school ordered fifty new grammar books.
That man makes bird cages.
Mrs. Taylor bought some new baby clothes.We can use a number with N₁ to make a compound adjective. Since the number and noun make one unit, we use a hyphen to join them. Note that N₁ remains singular because it is being used as an adjective.
Examples:
a three-car garage
a ten-speed bicycle
a twenty-dollar bill
     If we use a number to refer to the second noun (how many), we do not use a hyphen:
two grammar books
five rose bushes
     We can use a number to refer to the second noun (how many) and a number with the first noun (compound adjective). In that case, there is no hyphen after the first number, but the second number is still used with a hyphen because it forms a compound adjective.
two ten-dollar billshttp://www.englishcorner.vacau.com/grammar/rules/nounadj.html