当"only"位于句首并修饰状语时，句子通常需要采用部分倒装结构。具体来说，就是将助动词、情态动词或be动词放在主语之前，而实义动词的位置保持不变。以下是一些例句：

1. Only in this way can you learn English well.

2. Only then did I realize that I was wrong.

3. Only when the war was over in 1918 was he able to get back to work.

4. Only by shouting was he able to make himself heard.

需要注意的是，如果"only"后面跟的是主语，则通常不需要进行倒装。例如：

She saw only seven men.

They arrived only yesterday.

此外，当"only"修饰的是状语从句时，该状语从句本身不需要倒装，只对主句进行倒装。例如：

Only after a bitter struggle was the aim achieved.

总结来说，"only"开头的倒装句结构为： Only + 副词/介词短语/状语从句位于句首时，主句采用部分倒装结构，即助动词/情态动词/be动词放在主语之前。