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.