she must go to the library at a quarter past one

she must go to the library at a quarter past one
“she must go to the library at a quarter past one”这句话是我在新概念的英语书上看到的但是我感觉有点不对那个go 是不是应该变成goes因为是第三人称单数,动词要发生相应变化,谁能回答我