The student glider pilot reported that while conducting an approach to runway 18, he noted that another glider was staged on the runway. The student made a radio call advising that he would land on the grass between taxiway A3 and A4. The instructor on the ground radioed to the student pilot to “land short”. The student pilot stated that he interpreted the instruction to land short of his intended landing point between taxiways A2 and A3 and initiated a forward slip. The instructor radioed to clarify that he meant to land short on the runway behind the other glider. The student pilot attempted to extend his glide to make his first intended landing point between taxiways A3 and A4. He stated that he did not think he could make the A3 and A4 taxiways and extended his dive brake and spoilers to land between A2 and A3. The student pilot reported that about that time a wind gust stopped and the glider lost airspeed and developed a significant sink rate. The glider landed hard which resulted in serious injury to the student pilot and minor damage to the glider. The towplane pilot located on the runway at the taxiway A3 intersection reported that he saw the glider lose its forward motion and drop in a flat attitude to the ground. Based on the pilot and witness statement it is likely that the glider stalled while landing. The pilot reported that there were no preimpact mechanical failures or malfunctions that would have precluded normal operation.