‘A’ is an article. It is used in front of singular nouns that are countable (count nouns).
- I wrote a letter yesterday.
‘An’ is used instead of ‘A,’ in front of count nouns that begin with a vowel (a, e, i, o, or u), or vowel sound.
- I wrote an essay yesterday.
‘A’ and ‘An’ are not used in front of no-count (indefinite) nouns.
- I saw a water in the sink. (incorrect)
- I saw water in the sink. (correct)
‘Some’ is often used with non-count (indefinite nouns), and plural count nouns.
- I saw some water in the sink. (also correct)
- I saw some books on the table. (plural count noun)
‘The’ is also an article. ‘The’ is used to denote a specific noun.
- I saw the paper you wrote for your English class.
The writer of this sentence wants the reader to know that he/she is referring to one specific paper.
‘The’ is also used the second time that the writer mentions a noun.
- Yesterday I saw a movie. The movie was about killer clowns from outer space.
The may be used with any noun:
- The book = singular count noun
- The books = plural count noun
- The water = non-count noun
- I saw the book you’re looking for.
- Some books fell off the shelf. I nearly fell over the books lying on the floor.
- I drank some water. The water was very cold.