It should be noted that "tu, teu, tua", i.e., the second person is not typically used in Brazil - in favor of "voce, seu, tua".
The second person is, however, used in much of south Brazil and parts of north Brazil and in Portugal when speaking to close friends or family. Thus, if your focus is mainstream Brazilian Portuguese, you may never have the need for "tu, teu, tua, teus, and tuas" although it is good to know they exist (just in case).
In Scripture (the Bible), the formal forms of you singular (tu) and you plural (vos) are often used but are considered obsolete just as "ye" is considered obsolete in the English King James Bible.
Maybe Duolingo is just being didactic, but this sentence is not common Brazilian Portuguese. você/seu is used all over Brazil, whereas tu/teu is used only in a few regions and Portugal, and yet they also use and understand você/seu. seu can also mean his/her (since it's the proper pronoum for the 3rd person) but in Brazil it's used only in formal/written language with its meaning.
o meu carro = my car
o seu carro = your car
o carro dele/dela = his/her car
o nosso carro = our car
o carro de vocês = your car (plural)
o carro deles/delas = their car
Other uses are:
o teu carro = your car [used (alongside seu) only in a few regions]
o vosso carro = your car (singular/plural. archaic (used in bibles, prayers...)
o seu carro = his/her car in formal/written language
They are quite different. "Você" is one of two ways of saying singular "you", the other is "tu" (not so common in Brazil, but very common in Portugal). There are also two ways to say "your" and the one that goes with "tu" is "teus" and the one that goes with "você" is "seus".
(Note, things are a little bit more complicated than that because the word for "your" depends on what you are talking about and whether it is masculine/feminine and singular/plural so there are really 8 words for "your": "teu/tua/teus/tuas" and "seu/sua/seus/suas" depending on your choice of "tu" or "você". Even more of a surprise, though, is that any of "seu/sua/seus/suas" can also mean "his/her/your (plural)/their" which can make some sentences ambiguous.)