When I was in the 7th grade, my English teacher, Miss Tharp, stood on her chair in the front of the class and in a loud voice said “‘a’ (she pronounced it “ay”) is not a word.”
We did not understand what she meant until she explained:
The first letter of the English alphabet is “a”—it is pronounced like the “a” in able.
The indefinite article “a” as in “a book”—it is pronounced like the “a” in about.
Unfortunately, information like this is not taught in elementary school today.