Is a prayer valid if the Quran read in it is not proper? Do the mistakes of letters and diacritical marks made in prayer invalidate prayer?

I performed a lot of prayers behind an imam who pronounced the phrase "ghayril maghdubi" as "gayril magdubi" in the chapter of al Fatiha and the phrase "wa yamna’unal ma’un" as "wa yamnaunal maun", without pronouncing the letter ‘ayn. He pronounced the phrase "wa amanahum min khawf " as "wa amanahum min hawf ". That is, he pronounced the letter ‘kh’ as ‘h’. Are the prayers that I performed behind him accepted?

Dear Brother / Sister,

In Arabic, there are some difficult letters whose points of articulation are very close to each other like (za) and (dad), (sad) and (sin), (ta) and (tha), (ha) and (kha), etc. According to the majority of fiqh scholars, prayer is not invalidated if one of the letters of the pairs above is read instead of the other. Fatwa is based on this view.

However, if it is done deliberately, prayer is invalidated according to Qadi Abu Hasan and Qadi Abu Asim. However, if it is done as a slip of the tongue, prayer is not invalidated. This is the preferred view. However, the former is deemed more appropriate in terms of fatwa since it is easier.

Thus, a person who cannot pronounce some letters will not be regarded as excused; therefore, he has to make effort to learn them. However, if a person cannot pronounce some letters though he tries very hard and cannot do it by any means, he reads the verses that do not contain those letters. If not, he performs prayers by pronouncing the letters as he can but it is not appropriate for him to become an imam. (Fatawa Qadihan, Fatawa al-Hindiyya) This is the sound view.

Accordingly, the prayer of a person who pronounces the words in the question as you mention is not invalidated and the prayer of a person who performs prayers following an imam who pronounces the letters like that is not invalidated either.

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