fain > gladly, willingly
2 But yet dared not for dread approach nigh, 3 But stood aloof, unweeting what to do;
aloof > apart unweeting > not knowing
4 Till, pricked forth with love's extremity,
pricked forth > spurred on, goaded
5 That is the father of foul jealousy, 6 He closely nearer crept, the truth to weet:
closely > secretly weet > learn, find out
7 But, as he nigher drew, he easily 8 Might scern that it was not his sweetest sweet,
Might > Could scern > discern (OED quotes this instance)
9 Nor yet her belamour, the partner of his sheet.