The new coach is known to be very hard on his players during training.

be 是系动词，表示"是"或"存在"的状态。 hard 在这里是形容词，意思是"严厉的、苛刻的"，形容教练对球员的态度。 on 是介词，表示"对...施加影响或作用"。 be hard on 是一个固定短语，意思是"对...要求严格"或"对...苛刻"。