当“only+状语”位于句首时，其后习惯上要用部分倒装。其中，only后的状语可以是副词、介词短语、从句等。
Only in this way can we learn English. 只有这样才能学会英语。
Only then did I understand what she meant. 只有到那时我才明白她的意思。
Only by shouting was he able to make himself heard. 他只有叫喊才能让别人听到他。
注意，在only后作状语的是从句时，从句不要用倒装，要部分倒装的是主句。如：
Only when it rains do you feel cool. 只有下雨时才觉得凉爽一点。
Only when he returned home did he realize what had happened. 当他回到家里时，才知道出了什么事。
Only when we landed did we see how badly the plane had been damaged. 我们只是在着陆之后才看到飞机损坏的严重程度。
From:https://www.zgghmh.com/CET-6/20007549.asp