In English grammar , An article is one of determiners which comes before noun and modify it to show whether noun refers something particular or not.It is independent of number and gender means it never changes on change of gender or number.
Articles can be divided into two category definite and indefinite article.
Definite Article (the)
Definite indicates something specific,If the noun is already known to listener or pointing to something very specific or unique then we should use definite article before noun. the is only key word used as an definite article.
Example : The Himalayas (The is used because it is known to all , there is only one Himalayas so no confusion at all)
Where to use use definite article(the) in sentences?
1. 'The' is used before noun if that noun(person or thing) is unique.
The prime minister of india visiting my district tommorow. (Primie minster word is common but country India makes unique )
NASA's Mars rover landed on the Red Planet on 5 Aug, 2012. (Red planet s only one and known to everyone that makes unique)
2. 'The' is used before noun if noun is already discussed in conversation.
Does he know where I keep the book in room? (Here listener knows about which book you are talking about)