The article 'a' is used before words that begin with a consonant sound, while 'an' is used before words that begin with a vowel sound. The word 'university' starts with a 'yoo' sound, which is a consonant sound, hence the correct article to use is 'a'. For example, we say 'a university' instead of 'an university'. This rule helps in maintaining the flow of speech and clarity in communication.