be essential to 对…必不可少；做某事实是必不可少（重要）
1.A be essential to B A对B必不可少(重要)
此时to是介词：
Water and food are essential to life.
水和食物对生命是必不可少的。
Money is not essential to happiness.
金钱对于幸福并非必不可少。
Agriculture is essential to a nation.
农业对一个国家非常重要。
The agreement is essential to spurring economic growth around the world.
这项协议对于促进世界经济的增长是至关重要的。
Cooperative activity is essential to effective community work.
要把社区工作做好，协作活动是必不可少的。
2.下面的结构时to是动词不定式标志
It is essential to 做某事实是必不可少（重要）
It is essential to keep the two groups separate.
将这两组隔开是完全必要的。
It was absolutely essential to separate crops from the areas that animals used as pasture.
将庄稼和放牧区分开绝对必要。
It will be essential to establish how the money is being spent.
查清楚这笔钱将作何用途至关重要。
It is essential to feel at ease with your therapist.
在治疗师面前放松下来是很重要的。
It is essential to build up the military option and show that this is not a bluff.
关键是要做好采取军事行动的准备，以表明这不是虚张声势。
It is essential to protect her，保护她非常重要。