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 saw the elephant at the zoo." Here, we're talking about a specific noun. Probably there is only one elephant at the zoo.

Not necessary in the beginning, it may start from A or any word. The is used when we wish to make something or some object specific or particular while telling about it. For example : There was a blind girl. ......The girl was also poor.

The is used in various ways to indicate a particular thing(s) or person (s) or animal(s) etc. known to both the speaker and the listener such as the tree (s), the dog (s) or the boy (s), to indicate importance of something as the God, to indicate the greatness of something or somebody as the Ashoka, the Emptiness and the United Kingdom, sometimes used when there is an article 'of' in a name as the United States of Americha.

Before a Proper Noun

We use 'the' to indicate one particular thing!!

In the beginning of a sentence..