There are only a few words that begin with a silent H in American English.

Hour (‘our): An hour passed hourly.

Honor (‘onor): It is an honor to meet an honorable person.

Herb (‘erb): An herb is used in an herbal remedy.

Heir (‘eir): An heir might own an heirloom.

Honest (‘onest): An honest person has honesty.

Since they start with a vowel sound, they use an instead of a.