倒装句为什么要加did这种助动词?

1.Not only did the prices increase,but their wages were raised as well.
2.Not until a week later did he receive the letter.
上面两句,是所谓的不完全倒装,即用疑问句的形式来完成,不是说一定要用did,如果原句中有其它助动词或情态动词或be动词,就将助动词按疑问句的形式提前.
Not only does the sun give us light but also it gives us heat.
Not only did he speak English correctly,but also he speaks it fluently.
Not only is this young man clever but also he is hardworking
还有,表祝愿的句子,原型是什么?
Long live Chairman Mao!
理解为：Chairman Mao live long.那要是给别人讲，怎么引入这个did或is的概念呢？比如Not only does the sun give us light but also it gives us heat.这句，是因为有gave这个词吗？简单地说，就是按照改疑问句的形式来完成倒装句的。句中有be动词，就将be动词提前，有其它助动词，就将其它助词词提前，没有的话，就按时态（需要），加do/does/did。