Those of you that are already comfortable with the differences between SER & ESTAR are ready for this: ESTAR-the definitive reference. I wanted to do a review of all its uses (ie, verb tenses) in all it's GLORY.
*In order of importance (use)
> eu estou (conversational= tô)
> você está (conversation = tá)
> eu estava (conversational = tava)
> você estava (conversational = tava)
This form is used so much when talking about almost anything that "was" in the past that most people think it is the "simple past" (preterite tense). But ESTAVA is the preterite imperfect tense. That's because it's almost always used to talk about something that was happening (when...). The difference is this: When you're talking about what you did last weekend you're describing action in the past that was on-going. You were at the park when other things were going on. When you're talking about the fact that you were somewhere you are describing an action that was, and then ended. You have been to the park.
> eu estiver
> você estiver
Brazilians mostly use ESTAR with the triggers quando, se & assim que:
> eu esteja
> você esteja
Here are by far, the most popular ways this is used (other examples):
> eu estive (conversational = tive)
> você esteve (conversational = teve)
You only need to remember to use this verb form when talking about a place that you (or someone else) has been. Note that the conversational form is the same as for the verb TER. Do not be confused by this!
> eu estivesse (conversational = tivesse)
> você estivesse (conversational = tivesse)
Always think of this tense as "were to be", like this:
> eu estaria
> você estaria
You already know the SER form (seria). The ESTAR version is much less used for reasons that I cannot explain.
> eu estarei
> você estará
Use the future tense to give a certain seriousness or certainty to the phrase, like this:
You would mostly use the past participle for composing the past form for "have been" like this:
Pull this one out when you really want to impress or confuse someone. It's rarely used verbally, but you might see it in a newspaper or a legal text.