Learn Portuguese (Portugal): Os teus sapatos estão nessa caixa.

nessa is a contraction of the preposition em (in/on/at) and the demonstrative essa (that). When you want to say 'in that', em and essa combine to form nessa. It ends in '-a' to match the feminine word caixa (box).

Why do we need Os before teus sapatos? Doesn't that translate to 'The your shoes'?

Yes, it literally means 'The your shoes'! In European Portuguese, it is standard practice to put a definite article (o, a, os, or as) before a possessive word like teus. Since sapatos is masculine and plural, we use Os.

Could I use naquela instead of nessa to say 'in that box'?

It depends on where the box is! You use nessa (em + essa) when the box is near the person you are speaking to. If the box is far away from both of you ('that box over there'), you would use naquela (em + aquela).

Why does this sentence use estão instead of são for 'are'?