Learn Portuguese (Portugal): A estrada vai pelo campo. - The road goes through the countryside.

pelo is a mandatory contraction. When the preposition por (through, by, for) is followed by the masculine singular definite article o (the), they combine to form pelo. Since campo is a masculine noun and we want to say "through the", we must use pelo.

I thought por meant "for" or "by". Why does it translate to "through" here?

The preposition por has a few different meanings depending on the context. When you are talking about physical movement or a route, por (and its contractions like pelo) translates to "through" or "along". So, vai pelo campo means it goes through the countryside.

Usually we use a or para with the verb ir (to go). Why not in this sentence?

You use a or para when you are talking about a destination (going to a place). In this sentence, the countryside is not the final destination; it is the route the road takes. We use por (contracted to pelo) to show the road goes through that area.

Does campo always mean "countryside"?