When is the prostration of forgetfulness (sujud al-sahw) obligatory when correcting an error in Al-Fatihah, or when making an error in a verse by dropping a letter or pronouncing it unusually and then correcting it? And what is meant by "changing the meaning" in Al-Fatihah?

If a worshipper makes a mistake in Al-Fatiha and corrects it, their prayer is valid, and no prostration of forgetfulness (sujud as-sahw) is required, according to the Malikis, especially if the mistake involved changing a letter or pronouncing it from the wrong articulation point.

As for a grammatical error (laḥn) that changes the meaning, if done intentionally, the prayer and the prayer of those behind him are invalidated by consensus. If done unintentionally, it is valid by consensus. If the person is naturally incapable (of correcting it), their prayer and the prayer of those behind him are valid. If the person is ignorant but willing to learn, there is a difference of opinion, but the preponderant view is that their prayer and the prayer of those behind him are valid. An example of a grammatical error that changes the meaning is pronouncing the "taa" in (an'amta) with a damma (an'amtu), as it changes the doer of the action.

As for a grammatical error that does not affect the meaning, the prayer is not invalidated by it, but it is disliked (makruh), and intentionally committing it is forbidden (haram). However, if one commits it intentionally, the prayer is not invalidated by it according to the preponderant view.

One should turn away from obsessive doubts (waswasah), for they are an ailment that corrupts religion and life. Their cure is to completely disregard them.