What is the ruling on praying behind an Imam who makes mistakes in his prayer, such as reciting Al-Fatihah before fully standing for the second rak'ah, and not prostrating on his toes, and is the prayer behind him valid?

Standing is a pillar in the obligatory prayer, and it is obligatory to recite the entire Al-Fatiha while standing, such that its recitation does not begin until the worshipper is fully upright. If he recites any part of it before that, his prayer is invalidated if he is able to stand. However, if he is unable to stand fully (like an elderly person), there is no harm upon him. As for prostration, it must be performed on seven limbs, including the tips of the feet. The most complete way is to prostrate on the bottoms of the toes, but if one prostrates on the tops of the feet, his prayer is valid, because the tips of the toes will be on the ground.