英语句子结构问题Three things in life that,once gone,nevercome back:tome,opportunity and words.句中这个,once gone,是什么结构?（插入语么）修饰Three things in life that的吗?为什么不能直接是作为从句Three things in life that once gone nevercome back

英语句子结构问题
Three things in life that,once gone,nevercome back:tome,opportunity and words.
句中这个,once gone,是什么结构?（插入语么）修饰Three things in life that的吗?为什么不能直接是作为从句Three things in life that once gone nevercome back