Learn Portuguese (Portugal): Elas abrem as janelas e limpam-nas.

When a verb ends in a nasal sound (like the -m in limpam, abrem, or an -ão ending) and is followed by the direct object pronouns o, a, os, or as, we add an n to the pronoun. So, limpam + as becomes limpam-nas. This rule exists to make the transition between the two words easier to pronounce smoothly!

What does the -nas refer to in this sentence?

It refers back to as janelas (the windows). Because janelas is feminine and plural, we use the feminine plural pronoun as (meaning "them"). Due to the nasal ending of the verb, that as simply transforms into -nas.

Wait, doesn't nas usually mean "in the" (em + as)?

Yes, but they are two completely different things that just happen to look the same! The word nas meaning "in the" is a preposition that goes before a noun (like nas janelas). Here, -nas is a direct object pronoun attached to a verb with a hyphen, meaning "them".

Why is the pronoun attached to the end of limpam instead of going before it (e as limpam)?