1.Only 副词（或介词短语、状语从句等状语成分）位于句首是，句子要进行主谓部分倒装：

Only then did I realize that the man was blind. 只是到了那时我才意识到他是盲人。

Only after many setbacks was success achieved.

几经周折才告成功。

Only after a year of friendly discussion did Ms ． Garza finally say yes ．

经过一年的友好讨论,加尔扎女士终于答应了。

Only in the reading-room can you find him. 你只有在阅览室才能找到他。

Only you study hard can you get good results.

只有肯努力，才能得到好的学习成绩。

《汉英大词典》

Only when I worked as an interpreter did I realize how important it was to pratise speaking English.

只有当我做翻译的时候，我才意识到练习说英语是多么重要。

Only after she died was I able to appreciate her. 只有到她死后我才认识到她的价值。

Only when one loses freedom does one know its value. 一个人只有在失去自由后才知道自由的可贵。

注意，only 状语从句时，主谓部分倒装的结构发生在主句里，这个状语从句还是正常语序。

Only when the telescope is inclined at an angle of thirty degrees can you see that star.

只有当这架望远镜以30度角倾斜时，你才能看到那颗星星。

《外研社英语同义词辨析词典》

Only when each logical step has been checked by other mathematicians will the proof be accepted.

只有每一逻辑步骤都被其他数学家验证之后，证明才能成立。

Only after much persuasion from Ellis had she agreed to hold a show at all

埃利斯劝了她半天后她才同意办一场展览。

Only after countless rewrites did John consider the script ready.

只有经过了无数次修改后，约翰才认为剧本写好了。

以上3例句摘自《柯林斯高阶英汉双解学习词典》

2. 若位于句首的不是only 状语，而是 only 宾语等，则通常无需倒装(但有时也可以倒装)：

Only the teachers are allowed to use this room.

只有老师才能使用这个房间。(only 主语，不倒装)

Only five men were hurt in the accident．

只有5人在这次事故中受伤。(only 主语，不倒装)

Only a policeman the children saw in the street.

孩子们在街上只看到一个警察。(only 宾语，不倒装)

Only one more point will I make. 我只再说明一点。(only 宾语，倒装)