In German, you address relatives, friends, children and other people you know well with the informal "du". But if you're addressing an adult you don't know well or figures of authority, such as police officers or your teacher, it is a sign of respect to use the polite form of address: "Sie".

You already know the verb form that "du" takes. Its conjugation is the 2nd person singular.

Example:
Verstehst du Traponisch?

The formal form of address, "Sie", takes the same verb form as the 3rd person plural. But remember that unlike the personal pronoun "sie" used for both the 3rd person singular and the 3rd person plural, the pronoun "Sie" to address someone formally is always capitalized.

Example:
Verstehen Sie Traponisch?