Question

Why is the definite article "the" used before some countries and not before some other countries?

Example: The Netherlands, the Vatican City, the UK, etc., but not the India, the France, the Germany, etc.

Answer 1

Generally, all prefix of united or archipelagoes of Island countries.

Explanation:

The USA, the UK, the Maldives, The Philippines etc need the definite article of THE before the country/countries.

But, North America, North Korea, South Korea like, direction ( an adjective before a country) to a country, we need not any definite article.

One single country name like: SWEDEN, POLAND, DENMARK,INDIA need not any article before them, these are basic rules of thumb that English grammarians have made a convention, simple to memorize.

But if you meant why --a history of it, sorry, I don't know--why. You can search it by google.

Many words or writing terms have histories too, now a different context, like
spelling Varsity vs University
Sincerely yours, adverb needs a verb/adverb/adjective--here it is absent, these have histories.

Answer 2

See explanation.

Explanation:

The definite article the is used with the geographical names in three cases:

• The names are plural

Examples: The Philipines, The Netherlands, The Alps and so on

• The names include common nouns

Examples: the United Kingdom, the Republic of Poland, the Baltic Sea and so on

• All names of oceans, seas, rivers have the

Examples: the Nile, the Atlantic (Ocean), the English Channel and so on.