为什么倒装句有部分倒装和完成倒装?

为什么倒装句有部分倒装和完成倒装?
完全倒装
原：The Second World War came in 1939.
倒：In 1939 came the Second World War.
原：Some advertisements about cars are there.
倒：Here are some advertisements about cars.
部分倒装
原：I think little he is a spy.
倒：Little did I think he is a spy.
原：Theory can be separated from practice in no way.
倒：In no way can theory be separated from practice.
部分倒装里为什么是把can这种助动词提前啊?