morrow > dawn uprear > rear (the prefix is an intensive)
4 His dewy head out of the ocean main,
main > {Broad expanse of water}
5 He up arose, as half in great disdain,
as > [as if] disdain > indignation, anger
6 And climbed unto his steed. So forth he went,
unto > upon
7 With heavy look and lumpish pace, that plain
lumpish > sluggish; low-spirited
8 In him bewrayed great grudge and maltalent: