Why can I not say o meu carro as seems to be correct in other situations? Why does Duolingo not show any rules when to use the article preceding the possessive pronoun or when not? It looks to me that i should throw dice every time.
In day to day speech that is pretty much how this sentence would be interpreted. If someone wanted to say "He showed me his car" they would probably say "Ele me mostrou o carro dele" but the original sentence is correct.