Answered by SunniPath Answer Service Team
Wa Alaykum Assalam wa Rahmatullah wa Barakatuhu,
In the Name of Allah, Most Merciful & Compassionate
From a previous question answered by Shaykh Faraz Rabbani:
A person forgets to recite another Surah (after Surah Faatihah) in the first two rakaats of a fard prayer. He should therefore recite another Surah in the second two rakaats and also make the forgetfulness prostration. If he forgets to recite another Surah in one of the first two rakaats, he should recite it in one of the latter two rakaats and also make the forgetfulness prostration. If he forgets to recite another Surah in the latter two rakaats as well, i.e. he forgets to recite another Surah in the first two as well as in the latter two rakaats, and only remembers at the time of reciting the at-tahiyyaat that he did not recite another Surah in one or two rakaats, the prayer will still become proper if he makes the forgetfulness prostration.
In a three or four rakaat prayer, a person forgot to sit down after the second rakaat. Instead, he stood up directly for the third rakaat. While standing up, if he is closer to the sitting posture, he should sit down and recite the at-tahiyyaat and thereafter complete his prayer and there is no need to make the forgetfulness prostration. But if he is closer to the standing posture, he should not sit down. Instead, he should continue with his prayer without sitting down. He should only sit down at the end. In this case, the forgetfulness prostration will become wajib. If after standing up, he still went back and sat down; he will be committing a sin and even in this case he will still have to make the forgetfulness prostration.
Wassalam,
SunniPath Fiqh Team