Salah for a person who cannot prostrate

Q 1: I am a seventy five year old man and my knees pain me that I can not make normal Sujud (prostration) on the floor. I can not bend my knees so I sometimes sit on a chair and do Sujud with the Imam (the one who leads congregational Prayer). When the Imam sits between the two Sujuds (prostrations) or for making Tashahhud (a recitation in the sitting position in the second/ last unit of Prayer), I sit on the chair. When he stands up I do so with him and when he makes Sujud I make so with him. Do I pray correctly? If not, what do I have to do? Provide me with your beneficial answer please. May Allah keep and safeguard you.

A: Due to the excuse which is mentioned in the question, it is permissible for you to sit on the chair while you are making Tashahhud and between the two Sujuds.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.