Definite article: The

 
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English has two articles: the and a/an.  
The is used to refer to specific or particular nouns; a/an is used to modify non-specific or non-particular nouns.

The definite article the is the same for all genders in singular and in plural and is used when the noun is specific or particular.  
example:
- The last electricity bill was too much! We'll need to change our old incandescent bulbs with new LED efficient lamps
Here, we're talking about a particular bill.

- The ferry has just left the port. We missed it! 
Here, we're talking about a specific boat, in a port

- I am watching the last documentary of National Geographic .
Here, we're talking about a specific documentary.

The can be used with non-count nouns, or the article can be omitted entirely.
example:
- He spilled the drink all over the floor 
(perhaps the drink I've just got him)
or He spilled water all over the floor (any drink)

We use the before:
- names of rivers, oceans and seas: the Volga, the Indian Ocean

- points on the globe: the Equator, the South Pole

- geographical areas: the Middle East, the West

- deserts, forests, gulfs, and peninsulas: the Gobi desert, the Persian Gulf, the Amazon rain-forest, the Italian Peninsula


NOTE:
Do not use the before:
- names of most countries/territories: Italy, Spain, japan; however, the Cook Islands, the Galapagos Islands, the United States

- names of cities, towns, or states: London, Paris, Berlin

- names of streets: Las Vegas Blvd., Oxford Street

- names of lakes and bays: Lake Victoria,  except with a group of lakes like the Great Lakes

- names of mountains: Mount Everest, Mount Fuji except with ranges of mountains like the Andes or the Alps or unusual names like the Matterhorn (the Cervino)

- names of continents: America, Africa, Europe

- names of islands Sardinia, Corsica, Madagascar, Cuba, except with island chains like the Cook Islands, the Hawaii, or the Canary Islands

Some common types of nouns that don't take an article are:
- Names of languages and nationalities: English, Italian, Spanish, Chinese (unless we are referring to the population of the nation: "The Argentinians are known for their barbecue")

-Names of sports: football, polo, baseball, hockey 

- Names of academic subjects: Maths, Biology, Chemistry, Physics, Geography, History



More examples, we say:

• in general: the sky, the sea, the ground, the country, the environment, the same, the cinema, the theatre, the bank the post office

• in a room: the radio, the light, the floor, the carpet, the window, the ceiling