An university?

Do you say a university or an university?

The rules on the use the indefinite articles a and an before singular countable nouns are easy to follow: use a before a word beginning with a consonant and use an before a word beginning with the vowel a, e, i, o or u.

These rules give us the following:

an apple, a bicycle, a cat, a doctor
an egg, a fish, a girl, a hamster
an ice-cream, a jar, a key, a lion, a motorcycle, a nail
an orange, a purse, a queen, a racket, a snail, a storm, a torch
an umbrella, a villa, a watermelon, a xylophone, a yawn, a zebra 

But perhaps the rules are intended to be broad and easy to learn, because they only tell us half the story. As you can see, applying them faithfully produces the wrong phrases:

Correct	Incorrect
an umbrella	*an university
a home	*a hour

What's going on here? Why are two of the examples wrong when they follow the indefinite article rules above?

Well, the answer lies in how the words university and hour are pronounced. In other words, the sound is more important than the spelling. To know how a word is pronounced, we can look up a dictionary. Here is an example:

hamster | ˈhæmstə |

(Online tools such as Text to Phonetics provide free transcription.)

The symbols between the two vertical bars give the pronunciation of hamster i.e. the phonetic transcription of hamster.

For this discussion, we need only to concern ourselves with the initial symbol or sound of the phonetic transcription. For hamster, this symbol is the letter h, a consonant sound.

Now let's look at the words umbrella vs university, and home vs hour:

umbrella vs university
umbrella | ʌmˈbrelə |
university | ˌjuːnɪˈvɜːsɪti |

home vs hour
home | həʊm |
hour | ˈaʊə |

The first phonetic symbol for umbrella is ʌ, a vowel sound, so we say an umbrella. For university, the initial sound is j, a consonant sound, so we say a university.

In the next pair, the beginning sound in home is the consonant h so we say a home. The word hour begins with the vowel (a diphthong¹) aʊ, so we say an hour.

1. A diphthong is one indivisible vowel sound that consists of two parts. The first part is the main strong component (the nucleus); the second part is short and weak (the glide).