What are the nullifiers of prayer (Salat), and does prayer become nullified by a simple mistake in Surat Al-Fatiha, and what is the interpretation of the Hadith: "Indeed, Allah has pardoned for my Ummah (community) their mistakes, their forgetfulness, and what they are forced to do"?

The nullifiers of prayer include errors in Al-Fatiha. If the error is minor and does not alter the meaning, the prayer is valid. However, if the error changes the meaning, or if a letter is replaced by another, or if a letter is dropped, then the prayer is not valid, unless the worshipper is unable to learn. The Hadith that speaks of lifting error and forgetfulness from the Ummah means that there is no accountability for the sin, not that the obligation is dropped. It is reported in Sahih Muslim on the authority of Ibn Abbas that Allah has lifted accountability from the Ummah for what they do out of forgetfulness or error.