Contemporary writers using American English generally follow these basic rules for using the indefinite articles a
- Use a before nouns starting with a consonant (a boy, a cat, a donut).
- Use an before nouns starting with a vowel (an apple, an elephant, an ice cream cone).
- For nouns beginning with a silent-h, use an (an hour, an honor, an honest man), but if the h is not silent, use a (a horse, a handle, a heart).
- Use a before nouns beginning with u and eu when they sound like "you" (a European, a university, a unit).
Writers of Early Modern English applied an more widely to words beginning with h because speaking patterns of the times made that initial letter silent in many words, such as an heroic act.
Now that the once-unspoken letter h is being heard in our modern-day language, use of an is becoming increasingly rare with harsh-h sounding words. An occasional an historical, an hereditary, or an hysterectomy may creep into formal writing — or even everyday conversation — but these dated references are going out of fashion.