Learn Polish: Idziemy aleją do parku. - We are walking along the avenue to the park.

In Polish, the Instrumental case is uniquely used with verbs of motion to indicate the physical path or route you are traveling along. So, idziemy aleją means we are walking 'via' or 'along' the avenue. You will see this pattern often, such as in iść drogą (to walk along the road) or iść lasem (to walk through the forest).

Why use idziemy instead of chodzimy for 'we are walking'?

Iść is a unidirectional verb of motion. You use idziemy here because the sentence describes a specific, one-way trip happening right now toward a single destination (the park). You would use chodzimy if this were a habitual trip, like 'we go to the park every Sunday', or if you were just walking around aimlessly.

Since a park is an open outdoor space, shouldn't we use na park instead of do parku?

It is a great rule of thumb that na is used for open spaces, but park is a common exception that takes the preposition do followed by the Genitive case. In Polish, you always go do parku (to the park) and spend time w parku (in the park).

Does the meaning change if I rearrange the words to Idziemy do parku aleją?