What is the ruling on praying the Subh supererogatory prayer before its time is due? And what is the exact time for the Subh prayer? And what are the books that discuss how to mend a prayer in congregation?

The time for the Fajr prayer begins with the appearance of the true dawn (al-fajr al-sadiq), which is the horizontal white light spreading across the horizon, not followed by darkness. It extends until sunrise. It is not permissible to rely on the false dawn (al-fajr al-kadhib), which is a vertical, pointed white light. The entry of the prayer time must be confirmed, relying on calendars that accurately reflect reality or on visual observation. One can also consult the scholars of the country for clarification. As for the question of whether the Fajr prayer should be performed when the stars have not yet disappeared or after their disappearance, the beginning of the Fajr prayer time is when the stars start to recede and set, and their traces remain until they disappear at the time of إسفار (when the light of dawn becomes widespread).