在英语中，当`only`位于句首并修饰状语（如副词、介词短语或状语从句）时，主句通常需要使用部分倒装结构。部分倒装意味着将助动词、情态动词或be动词等提前至主语之前，而实意动词的位置保持不变。
1. Only in this way can we learn English well.
2. Only when the war was over in 1918 was he able to get back to work.
3. Only by shouting was he able to make himself heard.
4. Only after a bitter struggle was the aim achieved.
请注意，如果`only`后面直接跟的是主语，则不使用倒装结构。例如：
Only he is responsible for the mistake.
以上信息应该能帮助你理解`only`在句首引起倒装的情况。
