Your phrase "جَزَاكَ اللَّهُ خَيْرًا" is already correct in Arabic. It means "May Allah reward you with goodness." However, if addressing different people, the correct forms are: جَزَاكَ اللَّهُ خَيْرًا (Jazakallahu Khayran) → For a singular male جَزَاكِ اللَّهُ خَيْرًا (Jazakillahu Khayran) → For a singular female جَزَاكُمُ اللَّهُ خَيْرًا (Jazakumullahu Khayran) → For a group (plural) All of these are correct depending on whom you are addressing.