it 作形式主语的用法
动词不定式的复合结构(for sb to do sth ) 作主语
 
如不明白请追问还有一个：He always goes swimming on hot summer days,but he can't swim well.的考点go doing sth 常用于“去从事一些体育， 娱乐活动”再如：go dancing/ hunting/ climbing/ skating/ boating...