| Explanation | Example |
|---|---|
| Before a singular countable noun: | 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 taken as an example representing all individuals belonging to a class: | 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 be or become | Ernest Hemingway was a writer. Isaac Newton became a great scientist. |
| In exclamations before singular countable nouns: | What a nice day! What a charming person! |
| Before few (used with countable nouns) and little (used with uncountable nouns) which imply "a small number" or "a small amount": | I have a few friends. (i.e. "several friends") I have a little fortune. (i.e. "some money" - positive meaning) |
| In expressions of price, speed, etc: | two pounds a dozen eighty miles an hour |
| Before Mr./Mrs./Miss + surname: | a Mr. Brown which means "a man called Brown" and indicates that he is a stranger to the speaker. |
| Before a proper noun to mean "someone having characteristics of the person named": | He was an Einstein of his time. Tom will never be a Nelson. |
| Before certain numerical expressions: | a dozen a thousand to denote 'one' and before such expressions as: a lot of a great deal |
| Before an uncountable noun when it is qualified by an adjective: | He has a strong character. Do you know that Robert Wilson has a good knowledge of Chinese? |
| Before the comparative form of an adjective which is followed by a noun: | This is a better strawberry. This is a more interesting book. |
| Before an absolute superlative. It means that an adjective is used in the superlative degree and is followed by a noun. In such a 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) |
