Final answer:
The appropriate form of the verb 'llegar' to fill in the blank in the given sentence is 'llegué'.
Step-by-step explanation:
The appropriate form of the verb 'llegar' to fill in the blank in the given sentence is llegué. The sentence in English translates to 'When I arrived this morning, you had already left.' In this sentence, 'yo' is the subject and 'esta mañana' means 'this morning'. 'Llegué' is the past tense form of the verb 'llegar' in the first person singular.
Learn more about Verb conjugation in Spanish