What should one do if they doubt the omission of a pillar like prostration or bowing, and what should one do if they forget a pillar before the prostration of forgetfulness?

If one forgets a pillar (rukn) of prayer, it is obligatory to perform it for the prayer to be valid; prostration of forgetfulness (sujood as-sahw) alone is not sufficient. Likewise, if one doubts having omitted a pillar, it is also necessary to perform it.

1. If the forgotten pillar is remembered before reaching its place in the next rak'ah: One returns and performs it and what follows it, then completes the prayer and performs sujood as-sahw. This is the preferred (rajih) opinion. 2. If the forgotten pillar is remembered after reaching its place in the next rak'ah: The incomplete rak'ah is nullified, and the subsequent rak'ah takes its place. 3. If the pillar is not remembered until after concluding the prayer with salutations (tasleem): One performs a complete rak'ah.

Doubt about omitting a pillar: Whoever doubts having omitted a pillar is considered as one who has omitted it. The preferred (rajih) opinion is that one returns to perform it, unless one has reached its place in the next rak'ah. However, if one predominantly believes they have performed it, then they do not return, and they must perform sujood as-sahw after the salutations.