Doblar vs. curvar vs. flexionar
The Spanish verbs doblar, curvar, and flexionar all translate to the English verb to bend, but they describe different physical actions. The correct choice depends on whether the object is being folded into a sharp angle, shaped into a smooth arc, or bent at an anatomical joint.
Doblar
The verb doblar is the most general and widely used word for bending. It implies folding, creating a sharp angle, or changing the direction of an object, and is commonly used for bending wire, folding paper, bending metal, or turning a corner.
El alambre es demasiado grueso para doblar con las manos desnudas.
(The wire is too thick to bend with bare hands.)
Tienes que doblar la hoja de papel por la mitad para hacer un avión.
(You have to fold the sheet of paper in half to make an airplane.)
El impacto del choque logró doblar el parachoques del coche.
(The impact of the crash managed to bend the car's bumper.)
Por favor no intentes doblar la tarjeta de crédito porque se puede romper.
(Please do not try to bend the credit card because it can break.)
Al llegar al final de la calle debemos doblar a la derecha.
(Upon reaching the end of the street we must bend to the right.)
El viento fuerte hizo doblar la antena de televisión del techo.
(The strong wind made the television antenna on the roof bend.)
Curvar
The verb curvar means to bend into a smooth, rounded shape or arc. It does not create a sharp crease or angle, and it is typically used when talking about roads, the natural bending of branches, or materials that are deliberately shaped into a curve.
El carpintero usó vapor caliente para curvar la madera de la silla.
(The carpenter used hot steam to bend the chair's wood.)
El camino empieza a curvar hacia la izquierda cerca del río.
(The road begins to curve to the left near the river.)
El peso extremo de la nieve hizo curvar las ramas del pino.
(The extreme weight of the snow made the branches of the pine tree bend.)
Es malo para tu postura curvar la espalda mientras trabajas en la computadora.
(It is bad for your posture to bend your back while working at the computer.)
El cristalero tiene una máquina especial para curvar el vidrio.
(The glazier has a special machine to bend the glass.)
Flexionar
The verb flexionar means to flex or bend something that has a natural joint or inherent elasticity. It is almost exclusively used in anatomy to describe the bending of body parts like knees, elbows, and muscles during movement or exercise.
Debes flexionar las rodillas al levantar cajas pesadas del suelo.
(You must bend your knees when lifting heavy boxes from the floor.)
El fisioterapeuta me pidió flexionar el codo lentamente durante la sesión.
(The physical therapist asked me to bend my elbow slowly during the session.)
Es muy importante flexionar las piernas correctamente al hacer sentadillas.
(It is very important to bend your legs correctly when doing squats.)
Si sientes dolor al flexionar los dedos debes ir al médico.
(If you feel pain when bending your fingers you should go to the doctor.)
El gimnasta logra flexionar su cuerpo con una facilidad impresionante.
(The gymnast manages to bend his body with impressive ease.)
Summary
In short, use doblar when bending materials into angles or folding them like wire and paper, choose curvar when shaping things into smooth arcs like wood or roads, and rely on flexionar when bending bodily joints like knees and elbows.