The one who substitutes one letter for another is known in Arabic as althagh (one whose pronunciation is defective). Several scenarios may apply in such a case:
1 – The defect in his pronunciation may be mild, so that he basically pronounces the letter, but not perfectly. In this case the defective pronunciation does not matter and he may lead the prayers.
It says in Tuhfat al-Manhaaj (2/285):
A slight defect in pronunciation does not matter, in the sense that the person basically pronounces the letter but it is not exact. End quote.
Al-Mardaawi narrated in al-Insaaf (2/271) that al-Aamidi said: If that – i.e., defective pronunciation – is mild, then it does not mean that the prayer is invalid, but if it is severe then the prayer is not valid. End quote.
2 – If the defective pronunciation is severe, such as substituting one letter for another, and the person is able to correct it but does not do so, in this case the person’s prayer is not valid and he may not lead the prayers, if that (mispronounced) letter appears in al-Faatihah.
Al-Nawawi said in al-Majmoo’ (4/359):
It is essential to recite al-Faatihah in prayer with all its letters, including those which are doubled (letters with shaddah) … if a shaddah is omitted or one letter is substituted for another even though the person is able to pronounce it, then his recitation is not valid. End quote.
He also said (4/166):
If a person whose pronunciation is defective is able to learn how to pronounce things properly, then his prayer is invalid in and of itself, and it is not permissible to follow him as an imam, and there is no scholarly dispute on this point. End quote.
Ibn Qudaamah said in al-Mughni (2/15):
Whoever omits a letter of al-Faatihah because he is unable to pronounce it, or substitutes another letter for it, such as pronouncing the ra’ as ghayn… if he is able to correct any of that and does not do so, then his prayer is not valid and neither is the prayer of anyone who follows him as an imam. End quote.
3 – If the defective pronunciation is severe, such as substituting one letter for another, but he is not able to correct his pronunciation, in this case his prayer is valid, according to scholarly consensus.
Al-Nawawi said in al-Majmoo’ (4/166):
If a person with defective pronunciation is not able to learn, because his tongue cannot pronounce the letters or there is too little time, and he cannot do that, then his prayer is valid in and of itself. End quote.
The scholars differed as to whether it is valid for such a person to lead the prayers or not.
Many or most scholars are of the view that it is not valid; others are of the view that it is valid.
In al-Majmoo’ (4/166), al-Nawawi narrated that the view that it is valid was favoured by al-Muzani, Abu Thawr and Ibn al-Mundhir; this is also the view of ‘Ata’ and Qataadah.
In Haashiyat Ibn ‘Aabideen (1/582) it is narrated that the view favoured by some of the Hanafi scholars is that it is acceptable for a person whose pronunciation is defective to lead the prayer.
These scholars quoted evidence for that, such as the following:
1 – The verse in which Allaah says (interpretation of the meaning):“Allaah burdens not a person beyond his scope”
If a person is unable to pronounce the letters properly, then he is not burdened with more than he is able to do.
2 – Analogy with one who is unable to stand. Standing is an essential part of the prayer without which an obligatory prayer is invalid, but it is waived in the case of one who is unable to stand and if he leads the prayer, the prayer is valid. The same applies to one whose pronunciation is defective leading the prayer, because he is unable to pronounce the words properly.
See al-Majmoo’, 4/166.
Ibn Hazm said in al-Muhalla (3/134):
As for the one whose pronunciation is defective or the one whose recitation is not clear, or the one with a foreign tongue (one who does not differentiate between daad and za’ or between seen and saad, and so on), or the one who makes many mistakes in grannar, it is permissible to pray behind them, because Allaah says (interpretation of the meaning):“Allaah burdens not a person beyond his scope”
So they are not expected to do more than they are able to, or anything that they are not able to. They have performed their prayers as they were commanded, and whoever performs his prayer as he was commanded, has done well, and Allaah says (interpretation of the meaning):
“No ground (of complaint) can there be against the Muhsinoon (good-doers)”
Shaykh Ibn ‘Uthaymeen was asked:
I heard someone say that it is not valid for a person whose pronunciation is defective to lead the people in prayer, i.e., it is not valid to pray behind him, because he has a fault. Is this correct or not?
This is correct according to some scholars, who think that if a person whose speech is defective substitutes one letter for another, such as substituting ghayn or laam for ra’, and so on, then it is not valid for him to lead the prayers, because he is like an illiterate man, and it is not valid for an illiterate man to lead anyone in prayer except others who are like him. Other scholars think that it is valid for him to lead others in prayer because if a person’s own prayer is valid, it is valid for him to lead others in prayer, and because he is doing what he is obliged to do, namely fearing Allaah as much as he can. Allaah says (interpretation of the meaning):
“So keep your duty to Allaah and fear Him as much as you can”
If an imam is unable to stand, he may still lead others in prayer who are able to do so, and this case is similar, because in both cases the person is unable to do something which is an essential part of the prayer – one is unable to stand and one is unable to recite properly. This view is the one which is correct: it is valid for one whose pronunciation is defective to lead the prayer, even if he substitutes one letter for another, so long as this is what he is able to do. But nevertheless we should choose a person to lead the prayer who has no faults, so as to be on the safe side and avoid an area concerning which there is scholarly dispute.Fataawa Noor a’al al-Darb
See also al-Sharh al-Munti’, 4/248, 249.
With regard to what the questioner mentions about substituting daad for taa’, if he really is substituting it with daad every time, then the ruling on this case is mentioned above. If this substitution is not obvious, then the defect in pronunciation may be mild and may not matter.
It may be that the questioner is being inappropriately harsh.