In English there are two articles: the definite article (the) and the indefinite article (spelled either a or an depending on the initial sound of the following word). Articles are used before nouns or noun equivalents and are a type of adjective.
The definite article (the) is used before a noun to indicate that the identity of the noun is known to the reader.
The indefinite article (a, an) is used before a noun that is general or when its identity is not known. There are certain situations in which a noun takes no article.
Examples of how and when to apply them:
The (before a singular or plural noun)
a (before a singular noun beginning with a consonant sound)
an (before a singular noun beginning with a vowel sound)
Count nouns - refers to items that can be counted and are either singular or plural.
Non-count nouns - refers to items that are not counted and are always singular
For the purposes of understanding how articles are used, it is important to know that nouns can be either count (can be counted) or noncount (indefinite in quantity and cannot be counted). In addition, count nouns are either singular (one) or plural (more than one). Noncount nouns are always in singular form.
For example, if we are speaking of water that has been spilled on the table, there can be one drop (singular) or two or more drops (plural) of water on the table. The word drop in this example is a count noun because we can count the number of drops. Therefore, according to the rules applying to count nouns, the word drop would use the articles a or the.
However, if we are speaking of water in general spilled on the table, it would not be appropriate to count one water or two waters -- there would simply be water on the table. Water is a noncount noun. Therefore, according to the rules applying to noncount nouns, the word water would use no article or the, but not a.
Use the article the when a particular noun has already been mentioned previously.
I ate an apple yesterday. The apple was juicy and delicious.
Use the article the when an adjective, phrase, or clause describing the noun clarifies or restricts its identity.
They are thinking about (the) closure of the bank.
The boy sitting next to me raised his hand.
Thank you for the advice you gave me.
Use the article the when the noun refers to something or someone that is unique.
The theory of relativity.
(The) distribution of wealth in the world in unequal.
Use no article with plural count nouns or any noncount nouns used to mean all or in general.
Trees are beautiful in the fall. (All trees are beautiful in the fall.)
We must promote teaching in schools.(In all schools we must promote teaching).