How were Chinese forces able to defeat the UN forces in the Korean War in 1950 and push them beyond the 38th parallel when the UN had air superiority, superior weapons and tanks, and experience dealing with human wave attacks from the Pacific War?

The Chinese forces were tactically skilled. The Chinese communist forces had well over a decade of experience fighting better-equipped enemies (mostly the Nationalist and the Japanese), and had developed methods to make up for their lack of heavy weapons.

"Human wave attacks" isn't an accurate description - Chinese attacks in Korea were nothing like Japanese banzai attacks. The Chinese in Korea made excellent use of terrain, darkness, and camouflage to get as close as possible to enemy positions, and if possible, to infiltrate into the enemy rear in order to cut off positions and attack them from all sides. Attacks made use of surprise as much as possible. Getting as close as possible compensated somewhat for their lack of heavy weapons, and if an attack failed, provided them some protection against air strikes. While the overall numbers on both sides of the battle might be similar, the Chinese frequently managed to gain local superiority in numbers, which helped make up for the firepower difference. This local numerical superiority, and Chinese persistence in continuing attacks despite heavy casualties, gave rise to the (inaccurate) idea of "human wave attacks".

These offensive tactics were enabled by effective patrolling to accurately locate enemy units and gaps in the enemy lines. If you don't know where the enemy is, you can't get as close as possible to them in the night. If you don't know where the gaps are, you can't infiltrate into the rear.

These infiltration tactics could and did result in panic in the enemy, who found themselves attacked by surprise and often from a flank and/or rear as well as the front. If they withdrew under the pressure of the attack, they might be ambushed by Chinese units of up to company size which had infiltrated into the rear. If the unit didn't withdraw, the attack would continue in an attempt to wipe out the defenders. If the defending unit either withdrew or was destroyed, the adjacent units could be easily attacked from the flank, and often from the rear.

Firepower through numbers rather than firepower through heavy weapons did result in heavy casualties when Chinese attacks failed, or when enemy artillery was quickly responsive.

While the Chinese forces had developed tactics to make up for their lack of firepower when attacking, these were of little help when defending. Not being able to come close to matching the UN forces in artillery was a major cause of Chinese casualties being so high.

The two main Chinese weaknesses were logistics and lack of defensive firepower. Lacking motorised transport, much of their supply was carried by pack animals, on bicycles, or on the backs of the soldiers themselves. Air interdiction made rail and road transport difficult, and movement during the day unsafe. This meant that large reserves of ammunition were rarely available, and the further south the Chinese forces advanced, the worse their supply situation became.

The Chinese supply limitations was one factor that affected the final front line position: if the Chinese tried to advance to the south, it became progressively more difficult, and if the UN/ROK forces pushed them north, it became easier for them to defend. Thus, the UN/ROK was able to stop the Chinese advance (once Ridgway stopped the US forces from retreating, and insisted on active patrolling to find out where Chinese units were (which helps a lot to avoid surprise attacks), and made sure that his forces made effective use of their considerable artillery superiority). The UN forces were then able to use their superior firepower to take advantage of the Chinese weakness when on the defensive to push them back to a suitable line to stop on while negotiations took place. The UN forces could have pushed further, but it would have become progressively more difficult, and more expensive in casualties. The line they stopped on was suitable terrain for a strong defensive position, and the next suitable positions to the north were about 120km further, which would have improved the Chinese supply situation by that 120km. Ridgway (by thi time the commander of the UN forces) asked Van Fleet (who had replaced Ridgway as commander of the Eighth Army when Ridgway replace MacArthur) to plan an operation to advance to a Pyongyang-Wonsan line. This he did, but he recommended not proceeding with it unless the enemy forces became weaker, or he was reinforced. That advance didn't take place, more due to armistice negotiations beginning, but Van Fleet had a good point: the further north he went, the stronger the enemy became.