The constant criticism from his boss was hard on him, affecting his self-esteem.

was 是 be 动词的过去式，表示"是"的状态。 hard 在这里是形容词，意思是"困难的、艰难的"。 on 是介词，表示"对...而言"或"在...方面"。 was hard on 这个短语表示"对某人来说很困难/难以承受"，常用来描述某种情况或行为对某人造成的精神压力或负面影响。