The imam of our mosque in the neighborhood makes obvious mistakes in reciting Surat Al-Fatihah and I tried to correct his mistakes, but sometimes he corrects himself and sometimes he makes the same mistake again. I do not say that my recitation is perfect, but it is better than his recitation. He also hangs his pants down to the ankles, and I tried to advise him about that, but he said that there is nothing wrong with that as long as he does not touch the ground, and I know that this is not true. Is my prayer behind him valid?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

You did not mention those mistakes that your Imam makes when reciting Al-Fatihah, so we could be able to decide whether it affects the validity of prayer or not. However, we have mentioned in previous Fatwas that if the mistakes in Al-Faatihah change the meaning, such as making the letter Taa' 'ت' in "an'amta 'alayhim" with a Dammah or the letter Kaf 'ك' in "Iyyaka na'budu" with a Kasrah, then the prayer is not valid with him, and the same ruling applies if he omits a letter in Al-Faatihah. As for the mistake that does not change the meaning of the word, like making the letter Dal 'د' in "Al-hamdu" with a Kasrah, it does not invalidate the prayer.

It is not a condition of the validity of the prayer that the person who is praying does not make Isbaal (wearing a garment that reaches below his ankles). The Prayer of a person making Isbaal (wearing a garment that reaches below his ankles) is valid, regardless of whether he is the Imam (the one leading the Prayer), the one being led in the Prayer, or the one performing the Prayer individually. The Hadeeth reported about the invalidity of his prayer is not authentic. The scholars differed regarding the ruling on Isbaal done without arrogance, and the majority of them held that it is not forbidden.

Allah knows best.