"only"后面接 状语时，通常需要使用倒装结构。具体来说，当"only"位于句首，并且后面跟的是副词、介词短语或从句等状语时，主句应该采用部分倒装，即把助动词、情态动词或系动词放在主语之前。
1. Only in this way can you learn English well.
2. Only when the war was over in 1918 was he able to get back to work.
3. Only then did he realize the importance of English.
4. Only by shouting was he able to make himself heard.
需要注意的是，如果"only"后面直接跟的是主语，则不需要进行倒装。例如：
1. Only you can understand this.
2. Only five men were hurt in the accident.
总结：
当"only"修饰状语时，主句需要倒装。
当"only"修饰主语时，主句不需要倒装。
