The list of vowels includes: a, e, i, o, u, and sometimes y.
A versus An:
The rules seem straightforward but, as in much of English Grammar, there are exceptions.
When a word begins with a consonant, precede it with a.
When a word begins with a vowel, precede it with an.
The exceptions come into play when a word, or an acronym (USB), begins with a vowel. For example, words that begin with u or y (and when the British use a word with a silent h) are tricky to handle.
Words That Begin With Y:
Y is sometimes a vowel. When the y is pronounced like a vowel you should use the an form. If the y is pronounced like y you should use the a form. For example, if a y is pronounced like an e, its use is as a vowel.
As a general rule, it is safe to assume that we do not precede words that begin with y with an. This is because most words that begin with y have a vowel as a second letter and this excludes pronouncing the y like a vowel.
Examples:
A Yeti appeared out of the mist.
She let out a yawn.
There was a yacht moored in the harbor.
Words That Begin With U:
The same principal that applies to words that begin with y also applies to words that begin with u. We do not precede words that begin with u with an unless the y is pronounced like a vowel.
Typically this is when the u makes the same sound as y in you:
The jury arrived at a unanimous decision. [pronounced like y]
Shaq felt that Kobe was a useless freak. [pronounced like y]
An unknown yacht was moored there. [vowel sound]
Words That Begin With O:
When a word that starts with o has the o pronounced like the w in won, use the a form. Otherwise it is treated as the vowel that it is.
It was a one-eyed one-horned flying purple people eater.
She was an orphan.
A hint about proper usage involves the principle that the only reason for using the an form is to prevent the a sound from stepping on the vowel sound at the start of the next word. The technical term for this is eliding, or causing us to omit the vowel sound at the beginning of the next word.
Acronyms:
If an acronym begins with a vowel sound it is treated as though it begins with a vowel and takes the an form. Otherwise it takes the a form.
A US Marshall brought them in.
Connect the cable to a USB port.
She finally attended an AA meeting.
Again, if there is a vowel sound at the start of the next word use the an form so as to avoid stepping on that vowel sound.
The Silent H:
This issue is one of things that separate the American and British use of the English language. Typically the unsounded h is preceded by the an form.
That was an honest mistake.
An honorable discharge was awarded.
Since words such as historic, habit, and hysteria begin with a silent h the British precede them with an because the second letter is a vowel.
An historic occasion... (British usage)
An habitual mistake... (British usage)
This is not typical American usage but we mention it here to explain this usage in the event that you have ever wondered.
More Examples:
If this seems complicated keep in mind that it’s mostly a matter of trying to avoid stepping on a following vowel sound. This applies for any word that follows with a vowel at the beginning or any that has a vowel sound at the beginning (the first letter is silent).