Learn Portuguese (Portugal): Os livros estão nas mesas.

nas is a contraction of the preposition em (in/on/at) and the feminine plural definite article as (the). Because mesas (tables) is feminine and plural, we use as, which combines with em to make nas.

I thought em (and its contractions) meant "in", but the translation says "on". Why?

In Portuguese, the preposition em is used for both "in" and "on" when talking about location. Whether it means inside a container or resting on a surface depends entirely on the context. Since books are typically placed on top of tables, nas mesas translates naturally to "on the tables".

Why do we use estão instead of são for "are"?

estão comes from the verb estar, which is used for temporary states and locations. Because this sentence is about where the books are located right now, we must use estar. The other verb for "to be", ser (which gives us são), is used for permanent traits, like what the books are about.

Why is the first word Os but later we see nas instead of nos?