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Everybody gave a little explanation and I should agree with then, so i should say that our portuguese came from the portugal portuguese and there they use this kind of structure, here we had mixed that original portuguese with a lot of others languages so whe do not use a lot of old portugtuese, it's correct to use TU, and if you do correctly it's not informal as people say, it's because people do not know how to use this correctly, "Tu foste me procurar?" "Você foi me procurar? " " was you lokking for me?" so the firt one is absolutly formal, and the second one is normal, but if you do not conjugate the verb in the right way "TU foi me procurar?" it becames really informal, and a quite bit bad on listening seens that you do not know a lot of portuguese or you are a diferent style of spech. in a short answer we do not use TU, because você is easyer! and everybody uses. :P