Indefinite Article in English

The indefinite article the is used in the following cases:

Explanations	Examples
With singular countable nouns:	I have a book. I can see a mountain.
When a singular countable noun is mentioned for the first time:	I have bought a house. Yesterday I met a friend.
When a singular countable noun is used as an example of an element representing all elements which belong to the group:	An elephant is heavy. (i.e. "any elephant" or "all elephants") In spring, a tree is green. (i.e. In spring, all trees are green.)
When a singular countable noun is used as a complement of a verb: be or become	Ernest Hemingway was a writer. Isaac Newton became a great scientist.
In exclamatory sentences with a singular countable noun:	What a nice day! What a charming person!
With few (used with a countable noun) and little (used with an uncountable noun) which denote "small number" or "small amount"	I have a few friends. (i.e. "several friends") I have a little fortune. (i.e. "some money" - positive meaning)
Expressions of price, speed etc:	two pounds a dozen eighty miles an hour
With Mr./Mrs./Miss + surname, when the person mentioned is unknown for the speaker:	a Mr. Brown which means "a man called Brown"
With a surname when we want to say that the person we are talking about has characteristics of the owner of the surname:	He was an Einstein of his time. Tom will never be a Nelson.
With certain numerical expressions:	a dozen a thousand = one thousand and with such expressions like: a lot of a great deal
With uncountable nouns preceded by an adjective:	He has a strong character. Do you know that Robert Wilson has a good knowledge of Chinese?
With superlative adjectives followed by nouns:	This is a better strawberry. This is a more interesting book.
With superlative adjectives followed by a noun. In this case the word most means ‘very’ or ‘extremely’:	Tom Smith is a most intelligent boy. (i.e. ‘a very intelligent boy’) Professor Brown gave a most interesting lecture. (i.e. a very interesting lecture)