calentaban
-they were heating
The word calentaban is the imperfect form of calentar in the third person plural.
See the full calentar conjugation

calentar

Bailarines nerviosos calentaban y practicaban sus movimientos.
Nervous dancers were warming up and practicing their moves.
Las habitaciones se calentaban con hornos de carbón.
The rooms were heated with coal stoves.
Algunas viviendas no tenían vidrios en las ventanas y se calentaban solo con una pequeña estufa de leña.
Some were without glass windows and were only heated by a small wood-burning stove.
Los romanos gustaban del confort, calentaban sus viviendas con vapor, lo que atestiguan los restos de ese sistema, así como del sistema de acueductos.
The Romans lived in comfort, their houses were heated by steam, and we still have the remnants of that heating system, as well as the aqueduct.
Los jugadores de críquet calentaban en la cancha.
The cricket players were knocking up on the pitch.
Los herreros calentaban los metales en la fragua con carbón.
The blacksmiths heated the metals in the forge with coal.
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