Final answer:
The subject pronoun used to address an adult you just met in Spanish is 'usted'
Step-by-step explanation:
The subject pronoun that you would use to address an adult you just met would be usted. In Spanish, usted is the formal singular pronoun used to show respect or politeness to someone you do not know well. It is equivalent to the English pronoun 'you' when addressing an adult.
Learn more about Subject pronouns in Spanish