Answer:
An and a are both indefinite articles that refers to a member of a particular group. We will use "an" before the words that begin with vowel sounds. On the other hand, we will use the "a" if the words before that begin with consonants sounds. Before the abbreviation and letters, we use a or an depending on how way you pronounce the word. It's important to know to correct and to enhance your grammar when you're writing.
Examples:
An apple is red.
I got an A in Math but a C in English.
Have you ever been to an NCL exam?
