语法是错的，首先是没有主语，而且前面作为非谓语，后面是不需要连词but的，再有就是在非谓语中，彩虹是主动消失的，而非被动，所以不应用 disappeared，而应用 disappearing，可以改成这样：Seeing the rainbow disappearing gradually, nobody can do anything to keep the happiness.意思是：看着彩虹慢慢消失了，却没有人能够将这份幸福保持下去
Seeing the rainbow disappeared, but can't do anything to keep the happiness.没主语，另外可以不用but,因为前面是seeing伴随状态~改完后Seeing the rainbow disappeared, I can't do anything to keep the happiness.
