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)
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