As defined by Oxford Dictionary, a noun is a word used to identify people, places, things, and ideas. Since a complete sentence in the English language requires a subject and verb, nouns are very important. The subject of a sentence is always going to be a noun; and therefore, is very important to understand its usage. See examples below:
Nouns as the subject of a sentence
a. The concert was at 8pm.
Concert is a thing; and therefore a noun.
b. The countryside is a beautiful place to live.
Countryside is a place; and therefore a noun.
Additionally, nouns can act as other distinct parts of a sentence. See the following for examples.
Nouns as the direct object of a sentence
a. Sharks are known as predators.
Predators are the receiver of the verb (phrase); thereby, is the direct object.
Note: Ask “Sharks are known as what?” The answer to “what” is your direct object.
Nouns as the predicate nominative
a. You are the sun.
Note: "The predicate nominative is used with the verb to be and all its forms: be, am, is, are, was, were and been. Think of the verb as an equal sign: What's on one side of it is the same as what's on the other side . . .” (http://grammar.about.com/od/pq/g/predicatenominativeterm.htm, 2013).
You = sun (predicate nominative)
Nouns as the object of a preposition
a. The house lies over the hill.
Note: Any phrase which starts with a prepositional word (i.e. in, over, under, before, to, etc.) and ends with a noun is a prepositional phrase).
Nouns as possessive or modifier
a. The Detroit Tiger’s team is strong this season.