It's the knowledge of first aid make a difference that when

有,而且让人看不懂.这句要改一下：
When someone was hurt ,it is the konwledge of first aid that makes a different. 当有人受伤,是急救知识起了重要作用.
强调结构it is 后是被强调部分.不能强调谓语部分,它可以强调主语,宾语,状语.