副词then放句首一定要倒装吗？可以不倒装吗？

副词then放在句首时不一定要倒装，可以不倒装。
倒装句是把谓语放到主语前面的句子，把谓语动词全移到主语前面，这叫做完全倒装，只把助动词和情态动词放到主语前面时，就叫部分倒装。
部分倒装可以分为下列几种情况。
一、否定字放在句子开头，句子要部分倒装，如：
Never is he late for school.
Never does he go to school late.
Rarely does he tell a joke.
Never have I seen such beautiful scenery.
Hardly had I sat in the chair when someone knocked at the door.
No sooner had he reached his house than it began to rain.
Not until I talked to him did I know that he was homesick.
Under no circumstances will I allow you to go there.
No hope did I have at that time.
Not a word did my father say.
Never again will we make such a mistake.
I haven't visited the North Pole, nor do I plan to.
二、only放在句首时，句子需要部分倒装。
Only in case of emergency can we use this exit.
Only when it rains do you feel cool.
Only if you enter a good college, will I accept you as a friend.
三、程度副词和频率副词放在句首时，句子也要部分倒装，如：
Well do I remember the promise that you made last year.
So boring a lecture was it that half the students fell asleep.
Often do we go for a walk.
I often donate money to charity, and so does my wife.
四、地点副词放在句首时，句子这时是完全倒装，如果主语是代词时，不用倒装。当谓语动词用了进行时，现在分词连同地点副词一起放在句首。
Here comes the teacher.
Over the wall threw a shower of stones.
Here he comes.
Standing in the doorway was a man with a gun.
五、表语置于句首，句子这时是完全倒装，不需要助动词。当主语为代词时，不用倒装。
Happy is the country that has no war.
Faint grew the sound of the bell.
Happy he is.
Right you are.
then不属于上述情况，所以它放在句首时不引起倒装。当句子较长的时候，then后面的主谓结构也可以发生完全倒装。

倒装是为了使句子生动形象但也可不倒装,那样就不态地道. 改完是这样的Then voices all shouting together came.

主语是代词就不用倒装。