Learn Croatian: Oni mogu plesati, ali ne mogu igrati tenis. - They can dance, but they cannot play tennis.

These are the infinitive (dictionary) forms of the verbs, meaning "to dance" and "to play". When you use the modal verb moći (can), the action that follows is always in the infinitive. This is very similar to English, where we say "they can dance" instead of "they can dances".

How does the negative form ne mogu (cannot) compare to other common verbs?

To say "cannot", you simply put the word ne in front of the verb: ne mogu. This is easy compared to some other verbs you might remember where the negative word merges into one word, like "to have" (nemam) or "to want" (neću). With moći, it always stays as two separate words.

Do we have to use the word Oni (they) at the start of the sentence?

No, it is optional! In Croatian, the verb form mogu already tells us that the subject is "they". You could drop it and just say Mogu plesati... and it would be perfectly correct. We include Oni here mostly for emphasis or to make it absolutely clear who we are talking about.

Is tenis (tennis) in a specific grammatical case here?

Yes, it is in the accusative case because it is the direct object of the verb igrati (it is the thing being played). Luckily, for masculine nouns that are objects and not alive, like tenis, the accusative case looks exactly the same as the basic dictionary form.