Rule #1: Use the Definite Article ‘The’ with Countries that are States, Unions, Republics, etc.

We use ‘the’ before countries that contain a word like ‘Union’, ‘Emirates’, ‘Kingdom.’ These words mean that the country is a group of smaller states.

• the United States
• the Republic of Ireland
• the Czech Republic
• the United Arab Emirates

We also use ‘the’ before countries that end in a plural ‘s’.

• the Philippines (= the full name is The Republic of the Philippines)
• the Bahamas (= the full name is The Republic of the Bahamas)
• the Netherlands

Rule #2: Use the Definite Article ‘The’ with Names of Rivers, Seas, Oceans, etc.

We say the following:

• the Nile / the Nile River
• the Caspian Sea
• the Pacific / the Pacific Ocean
• the Mediterranean / the Mediterranean Sea
• the Panama Canal

Rule #3: Use the Definite Article ‘The’ with Deserts

• the Sahara / the Sahara Desert

Rule #4: Do Not Use ‘the’ with Lakes or Mounts

• I live by Lake Ontario.
• I swam in Lake Superior.
• He can see Mount Fuji.
• She can see Mount Rushmore.

Rule #5 – Use the Definite Article ‘The’ with Mountain Ranges

Just like how we add ‘the’ to countries that end with a plural ‘s’ (the Philippines), we add ‘the’ before mountain ranges (which also end in a plural noun).

• the Rockies / the Rocky Mountains
• the Himalayas / the Himalayan Mountains 

Rule #6 – Use the Definite Article ‘The’ with Building Names

We usually use ‘the’ before the names of buildings.

• the Emperor’s Palace
• the Tower of Pisa
• the Louvre
• the Pentagon
• The Marriott / The Marriott Hotel

This is not true, however, in some cases:

• The names of stations: Grand Central Station, Main Station
• The names of airports: Pearson Airport, Gatwick Airport
• The names of universities (without ‘of’): Columbia University, Santa Monica College

General Rule: Use the Definite Article ‘The’ with Names that Have the Preposition ‘Of’

• the Island of Lesbos
• the University of Toronto
• the Republic of Congo
• the Gulf of Mexico