Is the prayer of one who recites a short surah, forgetting to do so in the third rak'ah, valid after performing the prostration of forgetfulness, or must it be repeated? And is it permissible to pray on uneven ground such that the feet do not fully touch the ground, or where there is something preventing the feet from reaching the ground?

The prayer is not invalidated by reciting more than Al-Fatihah in the last two cycles (rak'ahs) of a four-cycle prayer or the third cycle of the Maghrib prayer, and prostration of forgetfulness (sujud (definition) al-sahw) is not obligatory for that. There is a difference of opinion regarding the permissibility of performing sujud al-sahw in such a case. As for praying on an uneven surface that prevents the foot from being placed in its usual position, it does not invalidate the prayer, but it contradicts the desired complete equilibrium and is disliked, similar to a worshiper standing on one leg or pressing their feet together.