There are two articles in English - the and a/an .

Use when something is already mentioned.

Use when talking about new things.

Use when both speakers know what they are talking about.

Use when talking about being a part of a group (occupations, nationality).

Use when the meaning is 'one'.

Use before seas, rivers, hotels, museums, newspapers.

Use with superlatives.

We don't use articles before plural nouns when talking about nouns in general.

We don't use articles before countries, towns, languages.

We don't use article before some places - 'at home', at/to school', 'at/to work', in/to bed'.