1.第一个句子,
为什么“Here is + 主语”这种倒装结构变成here it is就不能倒装,其原因是：如果主语是名词,那么就必须部分倒装,如：Here is your key.Here comes the bus.There has my sister been to.但是当主语是代词时,那么就成了你说的句子,如上述三句话改为代词：Here it is.Here it comes.There she has been to.倒装的主要目的是为了保持句子平衡,名词（有时候是名词短语）较长,置于动词前就会使句子显得头重脚轻,而代词组成字母较少,所以不倒装.
第二个句子：
条件虚拟语气在省去if时,必须要求部分倒装.如：
Were I you ,i would try again.(就是If I were you,I would try again.)
Should he do such a job,...(就是If he should do the job,...)
Had I been here....(就是If I had been here)
3.其他倒装：
主要出现在否定词或半否定词提前的情况下,如：下列词提前：
no,neither,hardly,no sooner,not only,not until,only,few等.
当然其他情况也有,这里不再一一列举.