Retrieve and diagnose
Articles: a before a consonant sound, an before a vowel sound, for a singular countable noun mentioned without a specific identity: a castle, an island, a university (/juː/ starts with a consonant sound). The identifies something already known or unique in context: “I saw a castle. The castle was old.” Use the in names such as the United Kingdom and the United States; normally no article before Australia or Canada. Do not infer an article from a letter alone.