为什么一个要倒装,还有一个陈述语序?

"Not only + 分句,but also + 分句"句型中的前一分句要部分倒装.
如：Not only does John love Chinese,he is also good at speaking it.但not only...but also...连接主语时,不倒装.如:Not only the mother but also the children are sick.
这是一个语法原则.