only 后接“状语”位于句首时，句子必须倒装。如：

Only then did she tell him about the attack.  直到那时她才把袭击的事告诉他。

Only in Paris do you find bars like this. 你只有在巴黎才可会找到这样的酒吧。

另外，如果 only 后面接的是“主语”的话，那就不存在倒装不倒装。如：

Only two men were saved. 仅有两个男人获救了。

Only Tom knows the answer. 只有汤姆知道答案。

但如果是only后接“宾语”位于句首呢？要不要装？

比如：He bought only one dictionary. 若要强调宾语，将 only one dictionary 置于句首：

是 Only one dictionary he bought. 还是 Only one dictionary did he buy.