​英语难句结构剖析

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 为何用的三单形式？还有这句话是和什么并列的？
（我了解不应该总是纠结这种语法问题，但我一碰到不可以讲解的结构，就忍不住想找到答案，期望老师能解答）

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.

or为并列连词，covers your hair显然与前面的cover your skin并列。cover your skin的主语为that，指代bubbles，为复数。所以covers无疑为cover之误。

你既然能发现covers用的怪异，也应该可以了解它是不对的。