The conditions of prayer:
The conditions of prayer are the things that must be met before you begin the prayer, and if they are not met, then the prayer is invalid. If you remember that you forgot any one of the conditions, then the prayer must be repeated.
The pillars of prayer:
The pillars of prayer are the obligatory actions that must be performed in prayer such as if you leave one of them off, then the prayer is invalid. In the case that a pillar of prayer is forgotten, then you have to perform the prostration of forgetfulness (before you salaam out) in addition to repeating that pillar that you left off. If you remember that you forgot any one of the pillars in any of the rakah's, then the rakah in which you forgot the pillar must be repeated and made up. If you remember after you salaam out that you forgot a pillar without making it up by repeating the pillar and performing sajdah sahw, then you repeat the prayer.
The requirements of prayer:
The requirements of prayer are the things that if you leave them off, then your prayer is still valid (as long as they were not left off deliberately) and you do not have to repeat the requirement you missed, you just have to perform the prostration of forgetfulness.
The prostration of forgetfulness:
The prostration of forgetfulness (sajdah sahw) are the 2 prostrations you perform when you inattentively omit or add something in prayer.
When you omit something you perform the 2 prostrations before you salaam out.
When you add something you perform the 2 prostrations after you salaam out.
Ruling on forgetting sitting during the tashahuud:
If you are praying, and you stand up after one of the 2 sets of rakah's, forgetting the tashahud, then there is a certain way to go about to correct this. If you are in the process of standing up and you remember that you did not say the tashahud while your knees are still bent then you can sit down before your knees are fully straightened and say the tashahud. If you had already stood up, and you remember that you did not say the tashahud while your knees are already straightened, then you do not sit back down.
