Articles tell if something is specific.
Take the example of ‘apple. When talking about ‘apple, are you talking about one apple, any apple, or a specific apple? If the apple is specific (sometimes called indefinite), then it would be ‘the apple.’ When we need to know who or what we are talking about, i.e., being specific, we use the word ‘the.’ When you are not specific, i.e., undefined, and do not know which apple, then you would say ‘an apple. An undefined article is called an indefinite article. Indefinite articles are ‘a’ or ‘an.’
‘A’ or ‘an’ is only singular and refers to only one item. ‘The’ can apply to one or many things if you ask for ‘an’ orange you expect one orange. If you ask for ‘the oranges,’ you would expect more than one orange.
Definite and indefinite articles
“The” is the definite article – specific. For example, “May I have the apple.” You know which apple you will receive. Therefore the apple is definite (defined.) There is most probably only one orange. “The” is singular or plural, which means that you could say “May the apples,” you would receive more than one apple.
“An or A”
“A” or “an” is the indefinite article – non-specific. For example, “may I have an apple.” You don’t know which apple you will receive. Therefore the apple is indefinite. “A” or “An” is only singular; you cannot say, “May I have ‘an apples’.” Instead, you could say, “May I have some apples.”
The indefinite article changes depending on the vowel sound that comes after it. If what you are referring to starts with a vowel sound, you would use ‘An,’ for example, “an apple” or “an orange.” If what you are referring to does not start with a vowel sound, then you would use ‘A,’ for example, “a pineapple,” or “a plum.”
Sometimes vowels at the beginning of a sentence produce a consonant sound
Be careful with the letter ‘U’. Sometimes ‘U’ will create a consonant sound like the word ‘union,’ in which case you say ‘a union.’ Other times the ‘U’ will produce a vowel sound as in the word ‘Umbrella,’ in which case you would say ‘an umbrella.’ ‘An’ is more natural to pronounce when followed by a vowel sound. ‘A’ is more natural to speak, followed by a consonant. English is ‘lazy,’ and we tend to use words that are easier to say.
These books provide both study and reference material for English Grammar