A, An, The: A Practical Guide to English Articles

Use 'a' or 'an' when introducing something for the first time or when the listener cannot identify which one you mean: 'I bought a book.'

Use 'the' when both speaker and listener know exactly which thing is meant: 'The book was excellent.' Second mention almost always takes 'the'.

Use no article for plural or uncountable nouns spoken about in general: 'Books are expensive', 'Water is free'.

'An' is chosen by sound, not spelling: 'an hour', 'a university', 'an MBA'.

Fixed expressions must be memorised: 'go to school', 'in hospital' (British), 'play the piano', 'at night', 'in the morning'.