Roger H. answered 04/15/23
Skills & Motivation: Elementary Reading / Writing and Public Speaking
Native English speakers will notice that when saying "a" prior to a vowel sound, it takes more effort. It is easier and more efficient to use "an" prior to a vowel sound. Hence, the rule is the same. For example, "An apple a day keeps the doctor away," is used rather than "A apple". We use the "a" word prior to consonant sounds, like "a cat".
"The" is used for a specific noun rather than a general noun. For example, "The apple over there on the counter is delicious".