When do we use a definite article?

When do we use an indefinite article?
Do we use the definite article or the indefinite article in case of Lakes, River, Sea, Ocean, Desert, Country, Region, Mountains?

Let's clarify these 2 issues first: 

1. 1. A country in a plural form? Any relation to the articles?
   
   Ex: The Netherlands, The Maldives, The Philippines, ... 
2. 2. Name - Noun or Noun - Name? Any reference to the articles?
   
   Ex: Lake Michigan but Hoan Kiem Lake?, ...  

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