- The definite aricle is used before an uncountable noun or plural noun when it is followed by a phrase specifying exactly what is being talked about.
I like the music they play at Christmas.
- When a singular countable noun has been mentioned before you use the definite article because everybody already knows exactly what is being referred to.
He hired a room. The room was very small and quite dirty.
- the definite article is used for countries with a plural name or name of countries which contain a common noun
He lives in the United Kingdom and I live in the Netherlands