准确地说,是：如果only引导句子的状语/状语从句并放在句首时,该句必须采用部分倒装,即其结构变成类似一般疑问句一样,把be/助动词/情态动词提前.
如：Only at the time did I realize I was wrong.
但是,如果Only不是引导状语而提前时,则句子无须倒装.
如：Only you understand me.
此题only是修饰句子主语you,所以无须倒装.