同学你好，该知识点来自沪江网校《初中水平直达雅思6.5分【剑桥全能随到随学班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好！
不是只要only在句首就倒装哦，必须满足以下情况呢：
1.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。
2.在句首的only如果后面跟了副词、介词短语或从句等状语，则主句用倒装结构。如only后跟的是主语，就不用倒装结构了。
例如：Only when the war was over in 1918 was he able to get back to work.
Only in this way can you learn English well.
Only after a bitter struggle was the aim achieved.
Only the teachers are allowed to use the room.(不倒装)
Only five men were hurt in the accident.(不倒装)
3.注意，only和所修饰的状语如果不在句首，则不倒装。
例如：The boy was helped out only when his father arrived in a hurry.
Only then did he realize the importance of English只有在那时候他才意识到英语的重要性。
4.only then,是过一小会儿的意思，一般用在句首，引导倒装句。
例如：Only then did he realize the importance of English.
解析：在句首的only如果后面跟了副词、介词短语或从句等状语，则主句用倒装结构。如only后跟的是主语，就不用倒装结构了。
希望我的回答有所帮助！