This is a very important difference in Portuguese. Ser is used for permanent conditions. Eu sou americano. I will always be an American. Estar is used for temporary conditions. Você esta na cozinha. You are in the kitchen. If you leave the kitchen, you will no longer be in the kitchen.
Honestly, I'd like to know. In Spanish, you can often ask questions by inverting the verb and subject, (as in English and many other languages) So far I've noted almost every question in Portuguese seems to be able to be interpreted as either declarative OR interrogative. Is that right? Spanish, Es el libro allí.?... is clearly a question... but it seems in Portuguese it must be: O livro está lá? I find that confusing, wouldn't, Está o livro lá? interrogative?