It is easier to go downhill than _climb up hill

问题描述:

It is easier to go downhill than _climb up hill
为什么这里要填to?than在这里是介词吗?

than 前后格式一致,属于并列关系
再者,也可这样理解,把语序调整一下,就成了
To go downhill is easier than to climb up hill.
这句中,it's 作形式主语,不定式作真正主语