Grammar rule17

 17. Use the indefinite article a with words beginning with a consonant sound. Use the indefinite article an with words beginning with a vowel sound. see When to Say a or an

• a cat, a game of golf, a human endeavour, a Frenchman, a university (you-ni-ver-si-ty)
• an apple, an easy job, an interesting story, an old man, an umbella, an honorable man (on-o-ra-ble)