网友务必注意：

句子开头要大写。可数名词单数句中不能零冠词。of helmet 和 on head 均为错误。

difficult to do 既然是悬空不定式，就不能用被动，被动意味着“不悬空”。

以下两种表达任选一种：

The invention of helmets made it more difficult for soldiers to get shot in the head.

The invention of helmets made soldiers more difficult to shoot in the head.