Learn Portuguese (Portugal): As chaves estão nos bolsos.

nos is a contraction of the preposition em (in) and the plural masculine article os (the). Because bolsos (pockets) is masculine and plural, we combine em + os to make nos.

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

In Portuguese, we use the verb estar (where estão comes from) to talk about the physical location of things. We save the verb ser (where são comes from) for permanent traits or identities.

Is the singular of chaves just chave?

Exactly! For most words ending in a vowel, you just add an 's' to make them plural. Also, because chave is a feminine word, 'the key' is a chave, which is why 'the keys' starts with the feminine plural article As.