Learn Spanish (Latin America): Los peatones solían caminar tranquilos por el centro. - Pedestrians used to walk peacefully through the downtown.

Solían is the imperfect form of soler, a verb used to express things you "usually do." It is always followed by an infinitive (here, caminar). In the imperfect tense, it translates perfectly to "used to walk" or "would walk." While you could just say caminaban to mean they used to walk, using solían caminar strongly emphasizes that this was a regular, habitual routine in the past.

Why does the sentence use tranquilos instead of the adverb tranquilamente (peacefully)?

In Spanish, it is very common to use an adjective (like tranquilos) to describe the state of the subject while they perform an action. Because it's an adjective, it must agree with the subject in gender and number. Since los peatones is masculine and plural, we use tranquilos. You could absolutely use tranquilamente, but the adjective form is extremely natural and frequently used by native speakers.

Why do we use por instead of para in por el centro?

Por is used here to indicate movement through, around, or along a physical space. The pedestrians were strolling through the downtown area. If you used para, it would mean "toward the downtown," focusing on the downtown as a final destination rather than the area they were wandering through.