Switching from formal to informal in midsentence?
One of the practice questions I got was "Por favor escribe tu libro". Is it correct to switch from formal verb conjugation (escribe) to informal possessive (tu) in one sentence? Shouldn't it be "Por favor, escribes tu libro" or "Por favor, escribe su libro"?
There isn't such a switch in this sentence, because it's an imperative sentence. Thus the verb 'escribir' must be used in the imperative form 'tú escribe' not in the indicative form 'tú escribes'.
In this case 'escribe' isn't the usted conjugation of the verb but its imperative form. Look here: http://conjugator.reverso.net/conjugation-spanish-verb-escribir.html . The "usted imperative" would be 'escriba'