Articles are words that define a noun as specific or unspecific. They can be definite or indefinite. There are three articles in the English grammar, and they are A, An and The.
We use a or an with singular nouns only.
(e.g.) A girl, An orange
We use a with singular nouns and adjectives which begin with a consonant sound.
(e.g.) A computer, A unit (yu+nit), A wonderful artist
We use an with singular nouns and adjectives which begin with a vowel sound.
(e.g.) An artist, An M.L.A. (em.el.a), An honest shopkeeper
Words beginning with consonant letters do not always begin with consonant sounds.
(e.g.) Honour (sounds like – onour)
Similarly words beginning with vowel letters do not always begin with vowel sounds.
(e.g.) European (sounds like yu-ropean)
1.We use the when a person, an animal, a plant, a place, a thing is mentioned for a second  time.
(e.g.) I bought a book this morning. I am reading the book now.
2. We use the when it is clear to the listener or reader which person, animal, place, or thing  we are referring to.
(e.g.) The judge found him not guilty.
3.We use the when there is only one such thing.
(e.g.) The earth goes round the sun.
4.We usually use the before ordinal numbers.
(e.g.) I live on the third floor.
5.We use the before some proper nouns such as:
(e.g.) The Indian Ocean, The Arabian Sea
6.We use the before names of most buildings, landmarks, monuments and natural wonders.
(e.g.) The Park Hotel, The Taj Mahal
7.We use the before names of places containing "of".
(e.g.) The Republic of China.
8.The names of places ending in plurals.
(e.g.) The Andaman and Nicobar Islands, The Netherlands.
Some proper nouns are not preceded by an article.
  • the names of continents - Africa, Asia
  • the names of countries - Belgium, India
  • the names of towns and cities. – Tokyo, Chennai
  • the names of streets - Ritchie Street.
Some nouns can be counted and they are called as countable nouns; some cannot be  counted and they are called uncountable nouns.
We use "a" or "an" only before countable nouns.
(e.g.) A leaf fell off the tree. (countable)
         Rain can cause flooding (uncountable)
We use "the" with uncountable nouns, when it is clear to the reader which things we are referring to. We do not use "the" with uncountable nouns when we are talking in general. 
**(Uncountable nouns do not take the plural forms).
(e.g.) The rice in this super market is good. Rice is the staple food of Asians.
The word "some" can be used with both countable and uncountable nouns in the following  ways.
(e.g.) I want some apples.  I want some papers