notuntil 用于句首改为强调句主句的助动词为什么没有了

Question

Not until the teacher came in did the students stop talking.
It was not until the teacher came in that the students stopped talking.
强调句中为什么能把did省了

Answer 1

Not until the teacher came in did （这个did 是构成倒装句用的，也就是说，Not until 在句首是，后面的主句要用倒装句，这时需要一个助动词来达到倒装的目的）the students stop talking.
It was not until the teacher came in that the students stopped talking.
强调句中不是把did省了，而是根本就不需要助动词。

Answer 2

你写的前一个句子是属于部分倒装，句首not否定词，句子中的助动词，情态动词，系动词放在主语的前面。
第二个句子是强调句，句型是it is+强调部分+that/who+其它，句首不是否定词了，所以就不倒装，没有助动词了。