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.

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