English has three article choices: a/an (indefinite), the (definite), and zero article (no article at all). The choice depends on whether the noun is specific or general.

A / an - one of a group, when it does not matter which one. Use when the listener does not know exactly which one you mean, or when you introduce something for the first time. A before consonant sounds, an before vowel sounds - it is about sound, not spelling. "An hour" (h is silent). "A university" (sounds like yoo).

The - a specific noun that both speaker and listener know about. Either it was mentioned before ("I bought a book. The book was great."), or there is only one in context ("Pass me the salt." / "The sun is out.").

Zero article (no article) - used with uncountable nouns and plurals when talking about things in general. "I love music." / "Dogs are friendly." / "Water is essential." Do not add the here - "I love the music" means specific music, not music in general.

Fixed expressions that take no article at all: at home, at work, at school, at university, by car, by bus, go to bed, go to hospital (British), have breakfast/lunch/dinner.

A1 Grammar