Oh, boy. Strap in. This is some intense stuff. The answer to your question is buried in here: https://en.wikipedia.org/wiki/Hindustani_grammar#Verbs , but it's not going to be easy, especially because पीना is one of the few truly irregular verbs in Hindi. But it's worth looking up to understand the difference between, say, पी है and पिया.
Hey Artikyulet. I know I am out of turn here (completely different subject)... but is it possible that you could explain to me the difference between the use of 'tu' (you) and 'tum' (you)? I am not getting that ... nor how it effects what follows with verb forms. Much appreciated if you can do. Thanks ... Paul
Well, 90% of the Hindi I know, I learned from Duolingo or Wikipedia, but here goes: you may be familiar (heh) with something like tous/vous in French or tu/usted in Spanish, right? Just different levels of formality. Hindi just takes it one step further and has three levels. Tu (तु) is the most familiar level. You could think of it as "intimate" level: it should only be between very close family or friends. It is grammatically singulair, and in the tenses we care about, takes "है". Tum (तुम) is mid-level familiarity. It's what you'd say when you're being nice, but not trying to overstep your boundaries. In a grammatical sense, it's technically plural, so it takes its special verb form हो whether you're talking to one person or several. Aap (आप) is the genuinely formal pronoun, that you'd use when you need to be polite. It's the form you use when you don't know someone well or they're higher status than you. It is always plural, too, and you could think of it as being in the third person. That's why it always takes "हैं". Use it like you would use usted in Spanish, if you're familiar with that.