not to 和 don't 的区别。

谓语动词是实意动词时，用don't 对谓语进行否定；
而该句中并非是对谓语的否定，而是对不定式的否定。
分析：by our teacher 是插入语，没插入时是：we are often warned to waste precious time,
对不定式to waste 进行否定，直接在不定式前加not，就成了：
we are often warned not to waste precious time.
加入插入语就成了原句的情况：we are often warned， by our teacher, not to waste precious time.

温馨提示：答案为网友推荐，仅供参考

当前网址：https://verywind.cn/ee/3rvvjfr3j7fevvrxe3.html