What were some tactics of William, Duke of Normandy used to defeat Harold? And what made Harold lose?

He didn't do anything particularly brilliant. Harold's forces, probably through lack of discipline, broke their shield-wall, allowing William's cavalry and archers to become effective. Or perhaps they were ordered into pursuit, as the UN troops did in Korea before they realized there were a bunch of Chinese there. Had Harold's troops simply stayed in place when William's retreated, it might very well have changed history.