Nouns and articles

Nouns can be particular or general: the house, a house. The words the and a are articles, or, in more technical terms, determiners. A house can be any house, but the house is a quite definite building. When a noun begins with a vowel (a, e, i, o, u, and, occasionally, y) the indefinite article a becomes an for the sake of easier pronunciation - an apple, an elephant, an orange. Sometimes an is used before words that start with h, especially if the h is silent: an honorary degree. If the h is sounded a is the standard form: an ’otel, a hat.
Nouns can be singular or plural in number: cat, cats. In some cases es is added to make nouns plural: dress, dresses. Some nouns change their forms in the plural, without adding an s: foot, feet; man, men; mouse, mice; goose, geese Some nouns do not change at all in the plural: sheep, fowl.
There are also group nouns, called noun phrases. This means that two or more nouns, or a noun and an adjective, are put together to form what amounts to, or works like, one noun: football stadium, rock concert, orange tree. In each case certain nouns - football, rock, orange - are attached to other nouns, and each modifies or describes the second noun in some way to convey a different kind of object. A football and a football stadium are two entirely different things, though they both have to do with the same game.


Some nouns are one-of-a-kind names: Suez Canal, Elvis Presley, Empire State Building. Also called proper nouns, they are capitalized to set them off from general nouns. Sometimes adjectives (words that describe nouns) are also capitalized. This normally happens when the adjective is made from a proper noun, especially a place or person: American literature, English countryside, Elizabethan theatre.
Nouns are used in different ways: The dog barks. The man bit the dog. In the first case, dog is the actor, or the one that initiates the action of the verb. In the second case, dog is acted upon. In The dog barks, dog is the subject of the verb. In “The man bit the dog”, dog is the object of the verb.
Sometimes a noun is the indirect object of a verb: He gave the dog a bone. Bone is the direct object; it is what was given. Because it was given to the dog, dog is considered the indirect object of the action.

Nouns can also be objects of prepositions - words like to, in, for, and by - so the above sentence could read: He gave a bone to the dog. The words to the dog are called a prepositional phrase.

Some verb forms take nouns as objects: Drinking milk is good for you. In this sentence, milk is the object of the verbal form drinking. Such a combination of verb and noun is called a verbal phrase.
Nouns can show possession: The dog's collar is on the table. The collar is possessed, or owned, by the dog. All possession does not indicate ownership, however. In The building's roof is black, the roof is on, but not owned by, the building. Adding an apostrophe [ ' ] and an s to a noun shows possession: the cat's tongue, the woman's purse. If the noun is plural (stands for more than one thing) or already has an s, then often only an apostrophe need be added: the mothers' union (that is, a union of many mothers). The word of may also be used to show possession: the top of the house, the light of the candle, the Duke of Wellington.

source:http://babelnet.sbg.ac.at/themepark/grammar/noun.htm