Learn Portuguese (Portugal): O selo está na caixa.

In Portuguese, there are two verbs for "to be": ser and estar. We use estar (and its forms, like está) to talk about where something is located. Even if the stamp is permanently in the box, describing physical location almost always uses estar.

Where does the word "na" come from?

The word na is a combination (contraction) of the preposition em (meaning in, on, or at) and the feminine word for "the", which is a. Because caixa (box) is a feminine noun, em + a combines to make na.

Could "na caixa" also mean "on the box"?

Yes! The word em (and its combinations like na or no) translates to "in", "on", or "at". You have to rely on context. Here, "in the box" is the most natural translation, but if the stamp was stuck to the outside, it would translate perfectly to "on the box".

Why is the word for "the" translated as "O" at the beginning, but part of an "-a" ending in "na"?