The indefinite article in English is a/an. Before words that begin with a consonant sound, we use a and before words with a vowel sound, we use an. The word university begins with a consonant sound, so we say a university. Hour begins with a vowel sound, so we say an hour.
Many people believe that a is the underlying form, the form from which an is derived. The reason is that more words begin with consonant sounds than vowel sounds. Thus we can say that a has wider distribution.
However, this is incorrect. The indefinite article a/an always comes before a singular noun. When we say an orange, we refer to one orange. There is a close relationship between the indefinite article and the number one.
In German, Dutch, Swedish, Norwegian and Danish, the indefinite article is the same as the word for one. In German, ein Haus means a house and also one house. If it is important to emphasize number, the speaker can add more information to make this clear from the context or can add stress to the word ein to specify number.
Since we know that a/an is derived from one, we can conclude that the underlying form of the indefinite article is an. The phrase an house became a house because the nasal deleted before a consonant. In a phrase such as an orange, deletion was blocked.
The underlying form of the indefinite article is an, a word derived from one. The nasal deleted before a consonant, a common process in the languages of the world. This deletion of the nasal can be analyzed as both ease of articulation and simplification.