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A subject pronoun is what you call someone, for example you, him, and they. To greet your doctor you should use usted which means you but is only used formally in Spanish. To greet your best friend you would use tú because it also means you but it is informal and you would not talk to your best friend very formally. This is different from English because in English there is only one way to greet someone, ‘you’ while in Spanish there is an informal way ‘tú’ and formal way ‘usted’.
