These are basic rules which explain the use of articles.

We need to understand two things when deciding which article to use before a noun.

• The difference between ‘the’, and ‘an/a’
• The difference between ‘a’ and ‘an’

The rules about ‘the’ vs. ‘a / an’

1. Use ‘the’ when there is one particular thing you are talking about.
2. Use ‘a’ / ‘an’ when there are several possible things, and you’re talking about ‘any’ of them.

The rules about ‘a’ or ‘an’. This is actually a set of rules:

1. If the word following the article starts with a vowel, (a, e, i, o, u), then use ‘an’.
2. If the word following starts with any letter apart from a vowel then use ‘a’
3. If the word following starts with the letter ‘h’, there are rules about whether to use ‘a’ or ‘an’ that depend on whether the ‘h’ is silent. The word ‘honour’ will take ‘an’. For example, “It is an honour to meet you.” The word ‘house’ will take ‘a’. For example, “I live in a house.”
4. Words such as ‘European’ are pronounced with a ‘y’ sound and take ‘a’. For example, “I met a European girl in the park yesterday.”

The choice of ‘a’ vs. ‘an’ is merely determined by the initial sound (pronunciation if you like) of the word following the indefinite article. It goes beyond the basic vowels and the silent vs. aspirated h.

5. Consider umbrella and university. Two words starting with the vowel ‘u’, but we say ‘an umbrella’ and ‘a university’ (because the word university does not start with a vowel sound!).

Because the determining word is the word immediately following the indefinite article, we write ‘a university’, but ‘an old university’ (because the word old starts with a vowel sound).