Plagal modes have the prefix Hypo- before the name of the mode. Here the Finalis is not the lowest pitch but the note a perfect fourth above it.
So basically, these modes are precursors of a Major/Minor system and throughout the history there were several variations of such modes. From the middle of the 16th century, the number of modes increased to 12 (as described by Glareanus in his Dodecachordon, 1547). In his system, he added authentic Aeolian (from A) and Ionian (from C) modes and their plagal counterparts to the entire spectrum.
Practice writing these modes first from the white notes and later from the sharp or flat notes. Also try to play them on the organ. As soon as this becomes easy, you can start improvising on such modes. Tomorrow I will describe how you can do that so stay tuned if this topic is interesting to you.
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