What is the error in the phrase: "He had already finished studying at an university when he was called to join the army."?

The phrase contains an error in the use of the article "an." The word "university" begins with a consonant sound, specifically the "yu" sound, which requires the article "a" instead. In English, the choice of "a" or "an" is determined by the sound that follows the article. Since "university" starts with a "yu" sound, "a university" is correct. Therefore, the phrase should read "He had already finished studying at a university when he was called to join the army." This correction ensures grammatical accuracy and consistency in English syntax.