Learn Croatian: Putujem s partnerom u grad. - I am traveling to the city with a partner.

We use the Instrumental case here because of the preposition s (with), showing who you are doing the action with. For regular masculine nouns like partner, you add the ending -om to make it partnerom.

When should I use s versus sa for 'with'?

Both mean 'with', but you must use sa when the following word starts with the letters s, z, š, or ž (and sometimes ps or ks) to make it easier to pronounce. Since partnerom starts with a p, you just use the shorter form s.

Why is it u grad instead of u gradu?

The preposition u is followed by the Accusative case when it shows motion toward a destination (like traveling 'to the city'). It is only followed by the Locative case (u gradu) when describing a fixed location ('in the city'). Because of the motion verb putujem, we use the Accusative form grad.

The dictionary says 'to travel' is putovati, so why is it spelled putujem here?