这个句子是祈使句的否定句，即是：Don't eat or drink in the library.不要在图书馆吃吃喝喝。用should改写的话，就是补上主语（即是要求的对象），一般还原的时候用you，即是：You shouldn't eat or drink in the library.
You shouldn't eat or drink in the library.
