..но я не понимаю.
было:
Only when she sleeps, as now, I can do what I want.
стало:
Only when she sleeps, as now, can I do what I want.
в чем фишка?
Всем спасибо. Кажется ответ найден.
famulan В Cambridge Grammar of English это дается правилом:
When only modifies an adverbial phrase or clause in front position, subject and verb are inverted. This style is very formal and occurs mainly in writing:
Only in summer do you see people making such fools of themselves.
Only when we got home did we realise we’d left the keys behind.