Learn Polish: Nie mogę dojść z ronda do przystanku. - I cannot walk from the roundabout to the bus stop.

The prefix do- on a motion verb emphasizes reaching a specific destination. While iść means the general action of walking and pójść means "to go" or "to set off", dojść specifically focuses on successfully making it all the way to a point. Since the sentence is about the ability to successfully get from point A to point B on foot, dojść is the exact verb you need.

Why is it z ronda do przystanku? Do both prepositions use the same case?

Yes, both z (when it means "from" or "out of") and do (meaning "to") require the Genitive case here. It is a very common pattern in Polish to pair these two prepositions together to express moving "from [point A] to [point B]".

I know the dictionary word is rondo. Why does it end in -a here?

Because it must be in the Genitive case after the preposition z. rondo is a neuter noun, and neuter nouns that end in -o change their ending to -a in the Genitive singular.

Why does przystanek turn into przystanku? Where did the e go?