Breakdown of Mogli pojechać darmową trasą, tymczasem woleli płatną autostradę.
móc
to be able to
pojechać
to go (by vehicle)
woleć
to prefer
trasa
route
Questions & Answers about Mogli pojechać darmową trasą, tymczasem woleli płatną autostradę.
How does tymczasem work here, and could we use za to instead?
Here, tymczasem acts as a strong contrastive connector meaning "whereas" or "yet." It highlights a surprising discrepancy: they had a free option, yet they chose to pay. You shouldn't use za to here. Za to implies a balancing trade-off or compensation (like "The route was expensive, za to it was very fast"), which doesn't fit the logic of simply choosing an unexpected alternative.
Why is darmową trasą in the Instrumental case, while płatną autostradę is in the Accusative case?
The cases are governed by the different verbs interacting with those nouns. To express moving along or via a route, Polish uses verbs of motion like pojechać with the Instrumental case (darmową trasą). However, the second clause uses the verb woleć (to prefer), which simply takes a direct object in the Accusative case (płatną autostradę).
If I wanted to add the verb "to go" to the second part ("whereas they preferred to take the toll highway"), how would the grammar change?
You would add the infinitive pojechać after woleli, but you would have to change the case of the highway! It would become woleli pojechać płatną autostradą. Because you introduced the verb of motion along a route again, it requires the Instrumental case, overriding the Accusative that was triggered by just woleli.