This is a touchy subject. Phil's got the most correct answer, Ben's got a practical answer, and I have a problem with Peachey's answer. This kind of construction does happen, even though it's not proper, so Phil's correct there. Ben provides the practical advice that language is diverse and choices like this just affect which segment of the population you sound like. And although Peachey's right that you should probably avoid it, I take issue with the "poor education" comment. Although it's informal, I think very few people would assume someone was poorly educated if they said "Why you..." or "You eat yet?" in *informal* settings. Of course, don't write reports or present a speech that way. But if you're with friends or family, it doesn't sound like poor education. Phrases like "ain't" and "He don't" sound even more informal, and *may* come across as poorly educated, but sometimes they're appropriate. I am a well-educated, native English speaker who prides himself on his proper usage of the language compared to those around me, but even I will sometimes use these constructions in casual situations, specifically to sound more casual. Sometimes, using the absolute correct forms comes across as stuffy or clunky, especially in rural areas, like the one I live in. We change our language all the time to the situation we're in, so if you're surrounded by poorly educated people (or, at least, people who *sound* poorly educated), it might be appropriate to match their style. Just remember to use proper language when it's called for, such as in a job interview! Just be aware that language changes depending on your surroundings. Definitely learn the rules. But once you've learned them, don't be afraid to break them... appropriately.

I have just one more point to add to this interesting discussion. It's important to remember than even the most informal and ungrammatical-sounding language also has its rules. When we slip into casual speech, we don't 'forget about' rules. We simply adopt different ones.

While it's possible to do this, as Phil has explained well, you as a learner should avoid it. If you still have a strong accent from your first language, shortcuts like this will simply make your English sound worse... even comical. Also, using informal speech with anyone you meet just signals "poor education". I'm sure the same applies in the Portuguese-speaking world.

It may be a politically incorrect thought, but Vitor you should also realise that the phenomenon you described is as much related to class and social subgroups as it is to ethnic subcultures. The "bottom line" is - What or who do you want to sound like? Learners sometimes unwittingly take on the mentality of an adolescent who is ill at ease with adulthood and the world, and they keep on asking, "Can I ignore this rule and that?" or "Can I escape from this convention or that and still be OK?" The truth is: it's your own life and you are free to choose what or who you sound like. You can choose to sound like a thug or a prince. Nobody at the end of the day cares. It's your own life that you have to live yourself.

Great observation, Vitor. It's common to drop the auxiliary in spoken English when asking questions with "you." Please note that you still need to put the main verb in the correct form, as if the auxiliary had been pronounced: "You eat yet?" (Yes, I did) "You eaten yet?" -- Yes, I have. "You eating yet?" -- Yes, I am. (By the way, even though we often answer just "yes" or "no," I always insist my students practice the full form -- Saying "Yes, I have" provides almost as much practice as saying the full grammatically correct sentence. When they get into a real-life situation, they can decide which form is most appropriate.) The omitted auxiliary is always obvious from the context, and / or the form of the main verb (base form, past participle, present participle, whatever). Students also need to learn the true pronunciation of "could have," "couldn't have," etc, or they will be at a total loss to understand a real conversation. Again, these short forms are not regional dialects -- they're the natural expression of the fundamental workings of English phonetics. As far as omitting the "s" of the third person singular -- this is just my opinion, but I would consider that to be "sub-standard." The same goes for the use of "ain't" in all circumstances.