The general rule states that the first mention of a noun is indefinite and all subsequent references to this noun are definite and take the.
A man is walking down a road. There is a dog with the man.
Definite Article: the
The definite article is used before singular and plural nouns when the noun is specific or particular. The signals that the noun is definite, that it refers to a particular member of a group. For example:
"The dog that bit me ran away." Here, we're talking about a specific dog, the dog that bit me.
"I was happy to see the policeman who saved my cat!" Here, we're talking about aparticular policeman. Even if we don't know the policeman's name, it's still a particular policeman because it is the one who saved the cat.
"I saw the elephant at the zoo." Here, we're talking about a specific noun. Probably there is only one elephant at the zoo.
Two special groups of nouns are considered definite in reference even if they have not been mentioned in the preceding sentence or clause.
- The first group consists of nouns which refer to shared knowledge of the situation or context. For example, in Canada you can sayThe Prime Minister will arrive tomorrowbecause there is only one Prime Minister in Canada, and so it is clear to whom you are referring. Similarly, if there is only one hospital in the town, you can sayHe's been working in the hospital for two years.But you couldn't say this in Toronto, where there are many hospitals. You would have to name the particular hospital in your first reference to it:He's been working at Toronto General Hospital for two years. He says the hospital is in a financial mess.
- The second group consists of nouns referring to unique objects:e.g., the sun/the earth/the Pope/the sky/the equator
Count and Noncount Nouns
The can be used with noncount nouns, or the article can be omitted entirely.
- "I love to sail over the water" (some specific body of water) or "I love to sail over water" (any water).
- "He spilled the milk all over the floor" (some specific milk, perhaps the milk you bought earlier that day) or "He spilled milk all over the floor" (any milk).
"A/an" can be used only with count nouns.
- "I need a bottle of water."
- "I need a new glass of milk."
Most of the time, you can't say, "She wants a water," unless you're implying, say, a bottle of water.
