being exposed to the sunlight too long will do harm to one's skin.为什么开头不能用exposed 区别在哪

这个问题问得很好!
解决此问题,需要划分句子成分：
【Being exposed to the sunlight too long主语】【 will do harm to谓语】【 one's skin.宾语】
句意：在阳光下暴露太久有害于皮肤.
Being exposed to the sunlight too long 是【v-ing形式短语】,可以做作主语,表语,定语,状语等.在此句中作主语.
而exposed 是【过去分词短语】,不能做主语,但可以做状语,定语或宾补.要用过去分词,句子可以改写为：
Exposed to the sunlight too long,one's skin will be harmed.=
If one's skin is exposed to the sunlight too long,it will be harmed.
建议：此类问题要彻底弄清,最好看看语法书上面【句子成分】这一部分,再加上【非谓语动词】部分.
祝你学习进步,时时开心!O(∩_∩)O~~