An exchange: 'haben' and 'sein', 'Sie' vs 'du', infinitive clauses with 'zu'

Use 'du' (you, singular) for a friend, family member, person you know well, and 'Sie' for adults you address by surname. 'Sie' (you, formal) is written with a capital letter, but 'sie' (they) starts with lowercase. If 'Sie' begins a sentence (and hence has a capital letter) use other clues to help decide if 'Sie' means you or they. Modal verbs are followed by the infinitive, which goes to the end of the sentence. Other verbs use 'zu' plus infinitive. In 2-verb structures, the 2nd verb in the infinitive is at the end of the sentence.