这两个短语都是成立的,看你用在哪里.
We must face to the problem.= 我们必须面对问题.
Facing to the problem is a correct way.= 面对问题是一正确的方法.
供参考.不清楚,可以继续讨论!face to the problem 用在句首可以吗？To face to the problem is a correct way.动词不定式短语是可以放前面的，但是通常会这样写：It is a correct way to face to the problem.