To lather up means to put liquid on your body - soap or shampoo or what we would call shaving cream and move it back and forth - mix it with water until you have bubbles that cover your skin - this is to lather up - or covers your hair, to lather up with shampoo on your hair.
请问这句话中的cover 为什么用的三单形式？还有这句话是和什么并列的？
（我知道不应该老是纠结这种语法问题，但我一碰到不能解释的结构，就忍不住想找到答案，希望老师能解答）