首先要知道，句子用动名词（划线部分）作主语。仔细体会为什么对错：

Exposing to sunlight for too much time will do harm to one's skin. （误）

Exposing yourself to sunlight for too much time will do harm to one's skin. （正）

Being exposed to sunlight for too much time will do harm to one's skin.（正）