Articles ('a' and 'the'): General Rules
We don't use 'a' or 'the' with plural and uncountable nouns when we make generalisations.
I hate cats. (all cats)
Specific examples - THE
We use 'the' to talk about specific examples (just one, or many) from a group, when the listener knows which one we are talking about.
I hate the big cat from next door. (only this cat)
I hate the three cats which live next door. (only these cats)
Any of a group - A/AN
We use 'a' with singular nouns when we don't care or it isn't important which one we are talking about.
Can I have a cat? (any cat, just one)
What's the difference between "a" and "an"?
We use 'an' when a word starts with a vowel sound (e.g. 'an apple', 'an umbrella') and 'a' for other words (e.g. 'a dog', 'a tree'). You must think about the pronunciation, not the spelling. For example, we say a university because it is pronounced like 'you-niversity'. We say an honest man because 'honest' is pronounced 'onest'.
There are lots of exceptions to these rules though - see the other articles pages to learn more information.
From: The Tiny TEFL Teacher