First: Every Muslim has to learn the matters that are essential for a valid Salah such as the pillars of Salah, its obligatory acts, and conditions of Salah. It is desirable for a Muslim to learn the rest of the desirable supplementary matters in Salah such as Du‘a’-ul-Istiftah based on the saying of the Prophet (peace be upon him):
“Pray as you have seen me praying.”
Al-Bukhari) But whoever cannot
learn some supplications of Salah due to old age or illiteracy, his Salah is valid because of the Saying of Allah (Exalted be He):
Allâh burdens not a person beyond his scope.
And the Saying of Allah (Glorified be He):
So keep your duty to Allâh and fear Him as much as you can
, and the saying of the Prophet (peace be upon him):
“When I command you to do anything, do of it as much as you can.”
But a Muslim has to do his best with regards to observing the obligatory matters in Salah and repeat them until he masters them, even if after a while.Second: The person offering Salah should
adhere to the authentically reported invocations from the Prophet (peace be upon him) between the two prostrations. He may say: "O My Lord, forgive me" and repeat it or say: "O Allah! Forgive me, have mercy on me, correct me, guide me, protect me and provide for me."Third: The manner of
sitting in Salah whether Iftirash or Tawarruk are acts of Sunnah (whatever is reported from the Prophet) as authentically reported from the Prophet (peace be upon him). (Part No. 5; Page No. 320) If the person performing Salah can not do them because of a disease, old age, or the like, there is no blame on him and his Salah is valid.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.