Which word is incorrectly used in: "He had already finished studying at an University when he was called to join the army."?

The word "an" is incorrectly used in the sentence because it precedes "University," which begins with a consonant sound. The correct article to use in this case is "a," since "university" starts with a "yoo" sound, not a vowel sound. This is important because articles in English must match the initial sound of the following word, not just the first letter. Using "a" would make the sentence grammatically correct: “He had already finished studying at a University when he was called to join the army.”