How does one pray if unable to stand during prayer due to back pain, yet is able to prostrate?

The accountable person must perform the pillars of prayer. If a person has an ailment that prevents them from performing one of the physical pillars, they are excused in that regard and should perform what they are able to; this is based on the Almighty's saying: (So fear Allah as much as you are able), and the Prophet's (peace be upon him) saying: "When I command you to do something, do as much of it as you can."

Thus, whoever is able to stand in the obligatory prayer without severe difficulty must stand. And whoever is able to stand for part of it but not all, must stand until difficulty arises, for the inability to perform part of a duty does not negate the entire duty.

Whoever is able to perform bowing (ruku') and prostration (sujud) but not standing, must perform them as they are. Therefore, whoever prays on a chair due to an excuse, and is able to perform bowing and prostration in their original forms, must do so by rising for bowing and descending for prostration. If one is able to prostrate but not stand or bow, they should pray sitting, nod for bowing, and then descend to prostrate on the ground.

If one is unable to bend for bowing and prostration, they should nod, making the nod for prostration lower than that for bowing.