1 And forth he fares, full of malicious mind,
mind > intention; memory
2 To work mischief and avenging woe
mischief > injury, disaster; evil
3 Wherever he that godly knight may find, 4 His only heart-sore, and his only foe,
only > pre-eminent [the only one worth considering] heart-sore > cause of grief
5 Sith Una now he algates must forgo,
Sith > Since algates > after all; altogether
6 Whom his victorious hands did erst restore
his > [the Redcross Knight's] erst > lately