What is the ruling on the prayer of someone who made a mistake in Al-Fatiha out of forgetfulness and inattentiveness, and only realized this today? And how are the missed prayers to be made up, knowing that he does not know whether he made a mistake by pronouncing "an'amtu" with a dammah or not?

If a worshipper doubts whether he made a mistake in reciting Al-Fatihah, his prayer is valid. If he recited "an'amtu" (أَنْعَمْتُ) with a dammah on the "ta" by mistake and corrected it immediately, his prayer is valid. This is because an unintentional mistake in Al-Fatihah does not invalidate the prayer, even if it changes the meaning, but the recitation must be repeated. However, an intentional mistake that changes the meaning invalidates the prayer. If the mistake does not change the meaning, it does not invalidate the prayer.