Learn Croatian: Ne pijem kavu, nego čaj. - I don't drink coffee, but tea.

In Croatian, nego is used to mean but rather or instead. You use it when the first part of the sentence is negative (like Ne pijem kavu) and the second part provides the direct correction or alternative (čaj). If you just want to connect two separate contrasting ideas, you use ali.

Why is the word for coffee kavu instead of its dictionary form kava?

Because coffee is the direct object of the verb piti (to drink). When a feminine noun ending in -a becomes a direct object, it takes the accusative case, and the -a changes to -u.

If kavu changes its ending, why does čaj stay exactly the same?

Čaj is also a direct object here, but it is a masculine non-living noun. In the accusative case, masculine nouns for non-living things do not change their endings—they look exactly the same as their dictionary form.

Could I say nego pijem čaj instead of just nego čaj?

Yes, you could say Ne pijem kavu, nego pijem čaj, but it sounds a bit repetitive. Just like in English, it is very natural to drop the second verb when it is exactly the same as the first one.