Learn Croatian: Što je nad nama?

The preposition nad (above/over) takes the instrumental case when describing a stationary location, answering the question 'where?'. If the sentence described movement to a place above us, it would use the accusative case instead.

Wait, isn't nama also the dative and locative form of mi?

Yes! The pronoun mi (we) uses the exact same form, nama, for the dative, locative, and instrumental cases. In this sentence, we know it acts as the instrumental because of the preposition nad.

Could I use na instead of nad in this sentence?

You could, but it would change the meaning. nad means 'above' or 'over' (usually without touching), while na means 'on' (touching the surface). Što je na nama? would mean 'What is on us?'.

Why can't we put je at the end of the sentence, like 'Što nad nama je?'?

The word je is an enclitic, which means it is an unstressed word that strictly requires the second position in the sentence. It has to come right after the first word or phrase, which in this case is Što.