The external forces that modify the Earth's relief are erosion, transport, and deposition by various agents including temperature, wind, rainfall, rivers, the sea, glaciers, groundwater, and living things. These external forces erode, transport, and deposit materials over time to shape the Earth's surface features through processes like erosion, sedimentation, and relief modification. Key erosive agents that shape specific landforms mentioned include glaciers in Alaska, rivers in Las Médulas, and the sea in the White Cliffs of Dover.