not only的倒裝用法,後面主詞為何要緊跟在but之後?

Not only置於句首的標準句型:

Not only + 倒裝的主要子句, + but S (also) V

其中also可省略, 也可換成as well置於句尾, as well也可省略.
but也常被一併省略, 但正式文章我不建議這樣做, 因為會造成一句雙動詞的形式錯誤.

例:
Not only did I buy him a meal, but I (also) walked him home.
Not only did I buy him a meal, but I walked him home (as well).
我不但請他吃飯, 而且我還陪著他走回家.

Not only..., but also... 是他正確的使用方法
Not only..., but also... 是一種 Correlative Conjunction (相關連接詞)， 其他的相關連接詞有: either...or; neither...nor; whether...or; both...and
正確的造句:
Rome is not only the capital city of Italy, but also a very famous place in the world.
翻譯: 羅馬不只是義大利的首都，也是一個聞名於全世界的地方。

NOT ONLY放句首
正式文法為ALSO不可接在BUT後面，要先加主詞或動詞再加ALSO
但ALSO可省略

BUT很多非正式的文章也直接省略
但正式文章中只有ALSO可以省略