How is a forgotten prayer to be compensated for, and does greeting someone of the opposite gender invalidate ablution?

If someone forgets a pillar of prayer while performing it and can still rectify it, they should perform that pillar and what follows it. If rectification is not possible, they should cancel that rak'ah and bring a substitute for it, then perform the prostration of forgetfulness (sujud as-sahw). If they exit the prayer and the interval is not long, the same applies. The point at which rectification becomes impossible, according to the Hanbalis, is when one begins reciting in the subsequent rak'ah. According to the Malikis, it is upon commencing the bowing (ruku') of the next rak'ah. As for the ruling on a person in wudu' greeting a non-mahram of the opposite sex or vice versa, merely doing so does not nullify wudu' unless it is accompanied by the emission of madhy or maniy.