Indefinite articleThe indefinite article 'a' (or 'an' before a vowel sound) is used with a singular countable noun that is either mentioned for the first time or identified as one in a row of many similar objects. The indefinite article stems from the numeral 'one' and can often be replaced, without loss of meaning, by words 'one', 'any', 'some'.
Example:Give me a pen, please (one pen, any pen). We saw a house (some house). I have a difficult task (one of many possible tasks).
Absence of indefinite article with uncountable and plural nounsIn English, the indefinite article can only be used with countable nouns in singular. This means that, with uncountable and plural nouns, the indefinite meaning is conveyed by omitting an article altogether. Instead, indefinite pronouns 'some', 'any' may be used.
Example:Last night we had snow. Give me some coffee, please. You are our guest of honor. He waited for hours. She doesn't have any friends.
Exception:The indefinite article may be used with an abstract noun accompanied by a descriptive modifier: I felt a certain impatience. A dull anger rose in his chest.
Definite articleThe definite article 'the' is used in both singular and plural with nouns denoting objects and concepts already familiar to the listener or reader, or unique objects. The definite article has its origin in the demonstrative pronoun 'that' and can often be replaced by it. The decision about using a noun with the definite article is determined
1) by a specific situation or
3) by a limiting noun modifier, or
4) by the object's nature.
Example:1) The teacher left the door open (objects familiar to the audience).
2) He sat down on a chair. The chair felt quite comfortable (object introduced in context).
3) She went to the nearest/following/main/ only store. Bob told me about the movie he saw yesterday (modifiers unambiguously identify the objects).
4) The moon orbits the earth (unique objects).