可能你在理解上有点错误,这句话的从句是省略了主语,如果补充完整就可以不用ING形式,只不过那样比较繁琐,即原来的话应为:when you drink and eat outside ,you should kepp the garbage until you find a trash can因为前面的句子和后面的句子主语一致,所以省略
理论上是可以的