1.此处的it 指代前面的ice；2.skate on 短语中on 是介词,介词后可跟名词,动名词,或代词宾格等做介词宾语；3.最关键的是skate on 短语相当于一个及物动词后需要有宾语,否则不完整.综上所述,it 需要加.句子最主要的须...那如果是及物动词，后面要加名词吗，举个例子，谢谢E.G.You are listening to me now.(listen to 相当于一个及物动词）I visited the Great Wall last year.(visit vt. 否则句子不完整）不光是名词，也可接其他词：I enjoy listening to music.(动名词短语作enjoy vt. 的宾语）Please take me to the park.(人称代词宾格做take vt.的宾语)I hear you will be back in an hour.(hear vt. 从句做宾语）等等。祝进步。