Use of A and AN effectively in English

A/AN

If the word following begins with a vowel sound, the word you want is
"an": "Have an apple, Adam." If the word following begins with a
consonant, but begins with a vowel sound, you still need "an": "An X-ray
will show whether there's a worm in it." It is nonstandard and often
considered sloppy speech to utter an "uh" sound in such cases.

The same rule applies to initialisms like "NGO" (for "non-governmental
organization"). Because the letter N is pronounced "en," it's "an NGO"
but when the phrase is spoken instead of the abbreviation, it's "a
non-governmental organization."

When the following word definitely begins with a consonant sound, you
need "a": "A snake told me apples enhance mental abilities."

Note that the letter Y can be either a vowel or a consonant. Although it
is sounded as a vowel in words like "pretty," at the beginning of words
it is usually sounded as a consonant, as in "a yolk."

Words beginning with the letter U which start with a Y consonant sound
like "university" and "utensil" also take an "a": "a university" and "a
utensil." But when an initial U has a vowel sound, the word is preceded
by "an": it's "an umpire," "an umbrella," and "an understanding."