then middle-earth mankind's Guardian, eternal Lord, afterwards made—
Once the sky is created, God makes the earth. The sentence structure is
tricky again, with the subject and object in mixed-up places. "Mankind's
Guardian" is the subject doing the making and "middle-earth" is the
object being made.
Wait, God made Gandalf? One hymn to rule them all, one hymn to find them. One hymn to bring them all and in the
darkness bind them. Get your minds out of Tolkien, hobbit-freaks.
"Middle-earth" is our earth. It's also another kenning. For more insight, see "Imagery" below.