You really should pay attention to the Tips & Notes (accessible behind the light bulb in the lesson launch window of the web browser Duo). This shows up in three of them: To be sg., To be pl., and Thou. Quoting from the tips for Thou:
Recall that the ty forms of verbs and pronouns are used for addressing single individuals with whom we are "on a first name basis" or who are much younger than we are. Conversely, the vy forms are used both when addressing single individuals in more formal contexts and when addressing groups rather than individuals.
In Czech, the level of informality when using the ty address goes beyond just using the first name. Intermediate situations in which the vy forms are used along with the first name are possible. This is how most moderators on Czech (and other Slavic) forums at Duolingo approach their interaction with the users. In Czech, both ty and vy forms can occur with the first name address.
In terms of grammar, the vy address of single individuals presents something of an inconsistent blend. Verbs (in the only tense we know for now, the present) and personal pronouns take on the plural forms, but adjectives remain singular. An example is in order. Kateřino, (vy) jste krásná. (Kateřina, you are beautiful.) uses the plural jste but singular krásná, whether the optional vy is included or omitted.