What about comforter? Like the (thicker) blanket you keep on your bed, especially during the winter?
Is there any rule when to use Tu form and when Você ? Or you just use it interchangeably ?
Yes, you can use them interchangeably. When speaking, you can use TU with the same conjugation as VOCÊ (third person, singular), but when writing, you must conjugate it correctly. There is no problem at all to pick one over another. I myself use almost all the time TU, because I was born in northeast, and it's the most common form there. In southeast Brazil, the VOCÊ form is most common, but they are understood everywhere and you will not sound like a foreigner if you mix them. It's normal.
I lived in Brasilia for a very long time and traveled regularly throughout Brazil. Using "tu" with the 3rd person was very common in speech but quite often disparaged by certain people along with "nós vai" (or in the pronunciation "nóis vai") and "a gente vamos".
Personally I think learners should stick with the more formal "standard" grammatical forms until they reach a very advanced level as this will quite simply be less confusing for them to not have to consider when to "code switch" between the "correct" and informal versions. I would regularly use "Cara, tu vai...?" with my very close male friends but would never have considered speaking that way with certain other people with whom I had very different relationships yet would still address with "você".