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

would 是情态动词，在这里表示委婉的询问或建议，相当于"想要"或"愿意"。 rather 是副词，意思是"宁愿、更愿意"，表示在两个或多个选项中选择更偏好的那个。 would rather 是一个固定搭配，表示"宁愿、更喜欢"，用于询问或表达偏好选择。