Final answer:
The sacrospinous ligament separates the greater sciatic foramen from the lesser sciatic foramen.
Step-by-step explanation:
The ligament that separates the greater sciatic foramen from the lesser sciatic foramen is the sacrospinous ligament. The greater sciatic foramen is a large opening formed by the greater sciatic notch of the hip bone, the sacrum, and the sacrospinous ligament. The lesser sciatic foramen is formed by the lesser sciatic notch of the hip bone, together with the sacrospinous ligament and the sacrotuberous ligament.