Only + 状语位于句首需要倒装（此处状语包括状语从句、介词短语、副词）
以 only 引导状语位于句首引起的倒装
Only by shouting at the top of his voice was he able to make himself heard.
Only in this way can we achieve what we want.
Only in each afternoon does the university library open.
Only when she called me did I recognize her.
Only then did he realize he was wrong.