The scholars stated that the Sunnah is to recite quietly in optional prayers offered during the day, and when praying at night, he has the choice between reciting out loud or quietly, but reciting out loud is better.

You can say one of the following wordings for Tashahhud: 1- At-tahiyyatu Lillahi was-salawatu wat-tayyibat, As-salamu `alayka ayyuhan-Nabiyyu wa rahmat-Allahi wa barakatuhu. As-salamu `alayna wa `ala `ibadillah As-salihin. Ash-hadu allaa ilaha illallah wa ash-hadu anna Muhammadan `abduhu wa rasuluhu. 2- At-tahiyyatu Lillahi was-salawatu wat-tayyibat, As-salamu `alayka ayyuhan-Nabiyyu wa rahmat-Allahi wa barakatuhu. As-salamu `alayna wa `ala `ibadillah As-salihin. Ash-hadu alla ilaha illallah wahdahu la sharika lah wa ash-hadu anna Muhammadan `abduhu wa rasuluhu. 3- At-tahiyyatu Lillahi, Az-zakiyatu Lillahi, At-tayyibatu Lillahi, As-salawatu Lillahi. As-salamu `alayka ayyuhan-Nabiyyu wa rahmat-Allahi wa barakatuhu. As-salamu `alayna wa `ala `ibadillah As-salihin. Ash-hadu alla ilaha illallah wa ash-hadu anna Muhammadan `abduhu wa rasuluhu. To send blessings on the Prophet, you can say the following: 1- Allahumma salli `ala Muhammad wa `ala ali Muhammad kama sallayta `ala Ibrahim wa `ala ali Ibrahim, innaka hamidun majid. Allahumma barik `ala Muhammad wa `ala ali Muhammad kama barakta `ala Ibrahim wa `ala ali Ibrahim, innaka hamidun majid. 2- Allahumma salli `ala Muhammad wa `ala ali Muhammad kama sallayta `ala ali Ibrahim, wa barik `ala Muhammad wa `ala ali Muhammad kama barakta `ala ali Ibrahim fil-`alamin, innaka hamidun majid.

If someone is unable to sit in the Sunnah manner between the two prostrations or in the tashahhud, it is permissible for him to sit resting on his knees.

If the worshipper forgets the opening supplication and starts the essential part of reciting al-Faatihah, he should not go back to the opening supplication. But if he does go back, he does not render his prayer invalid, because that does not change the appearance of the prayer. For more details, please see the long answer.

It is not permissible to say ‘Allahu Akbaru Kabira’ or ‘Allahu akbar al-’Azim’ or ‘Allahu akbar al-Mu‘ti’ in the takbir in prayer. The one who does that has introduced a bid`ah but his prayer is still valid.