One way is by subglacial deformation. This is when there is slippage between the ice crystals which is called creep.
This is added to by partial melting of ice at depth so the water acts as a lubricant between the crystal surfaces along slip planes.
This often results in crevasses when faulting occurs if the creep cannot adjust to the stress quickly enough.
Another way is Basal slidingwhich occurs in temperate (warm-based) glaciers, due to the sliding effect of the glaciers over the bed rock.
a third way Plastic Deformation takes place due to the weight of the ice and the slope/gradient.
Melt water beneath the glacier acts as a lubricant between the glacier and the bedrock. It is most effective on relatively steep slopes, especially in summer.