What should the worshiper do if, upon saying the opening Takbir for prayer, he finds that the Imam has finished the opening supplication (Du'a al-Istiftah) and has begun reciting Al-Fatiha: should he complete the opening supplication or listen to the Imam's recitation?

If you join a congregational loud prayer while the Imam is reciting, you must listen attentively, and you are not prescribed to begin with the opening supplication (Istiftah), due to the saying of the Prophet (peace be upon him): "And when he recites, then listen attentively." If the Imam is silent, there are two narrations concerning it: one is that it is prescribed to begin with the opening supplication, and the other is that one does not begin. Sheikh al-Islam Ibn Taymiyyah chose to begin with the opening supplication without recitation, because listening serves as a substitute for recitation, unlike the opening supplication.