Learn Russian: Потом трасса проходит через исторический квартал. - Then the route passes through the historical neighborhood.

The prefix про- usually means "passing by" or "passing through." Combined with the base verb ходит (to walk/go), проходит means the route "goes through" or "passes through" the neighborhood.

Why do we use the imperfective verb проходит here instead of the perfective пройдёт?

Even though the sentence starts with потом (then), describing a step in a journey, the physical path itself is permanent. In Russian, when talking about where roads, rivers, or routes go, we treat it as an ongoing, continuous fact and use imperfective verbs like проходит.

Could I just use the basic verb of motion идёт instead of проходит here?

Yes, you could say трасса идёт через... and it would still mean the route "goes through." However, using the prefixed verb проходит specifically emphasizes the action of moving through or past an area, which pairs perfectly with the preposition через.

Why is исторический квартал in the accusative case here, and why didn't its spelling change?

The preposition через (through) always requires the accusative case. The phrase исторический квартал is masculine and inanimate (not a living thing). For masculine inanimate nouns, the accusative case looks exactly like the dictionary form, so the endings do not change.