There is only one difference between an old man and a young one:the young man has a glorious future .
翻译：在年轻人和老人之间只有一个区别：那就是在年轻人面前有个光辉灿烂的未来.
There is only one difference between an old man and a young one
这是There be句型.
the young man has has a glorious是one difference 的同位语.