do > [do the following]
4 Give no odds to your foes, but do purvey
purvey > provide
5 Yourself of sword before that bloody day:
of > [with a]
6 For they be two the prowest knights on ground,
be > [are] the prowest > [of] the bravest on ground > in the world
7 And oft approved in many hard assay,
approved > proved, tested assay > assaults; trials
8 And eke of surest steel that may be found