Which would you rather do - go swimming or play tennis?

would 是情态动词 will 的过去式，在这里表示委婉的询问或假设情况下的选择。 rather 是副词，意思是"宁愿、更愿意"，常用来表达偏好或选择。 would rather 是一个固定搭配，表示"宁愿、更愿意"，用于询问或表达偏好。