Sith > Since grief > suffering, pain
2 (For what do not you know?), of grace I pray,
of grace > as a favour
3 Pity our plaint, and yield us meet relief."
plaint > {Lamentation, complaint; statement of grievance made in seeking redress} meet > fitting
4 With that the prophet still awhile did stay,
still awhile did stay > (Two interpretations are possible: (1) "motionless (or quiet) awhile did remain"; (2) "yet awhile did hold back" [delay putting them out of their misery])
5 And then his spirit thus gan forth display:
gan > did display > unfold, expound 6 "Most noble virgin, that by fatal lore