not only … but (also) … both … and …: He not only read the book, but also remembered what he had read.
Page 1041, Oxford Advanced Learner’s Dictionary
not only … (but) also … used to emphasize that sth else is also true: She not only wrote the text but also selected the illustrations.
Page 1037, Oxford Advanced Learner’s Dictionary
I normally use both words (‘but also’), but I sometimes omit the word ‘also’.
I would say it is not possible to omit ‘but’ in either of the two sentences you posted. In my opinion, the example given for “not only…(but) also” was not an appropriate example of a sentence in which the word ‘but’ could be omitted.
You might see the word ‘but’ omitted in a sentence such as this:
She was not only hungry, (but) also tired.
Not only was she hungry, (but) also tired.
As I said, however, I generally use both words (‘but also’), and I would include ‘but’ in the sentences above. Maybe someone else will have a different take on this.
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dictionary.cambridge.org/define. … &dict=CALD
not only … (but) also
used to say that two related things are true or happened, especially when this is surprising or shocking:
Not only did he turn up late, he also forgot his books.
If this project fails it will affect not only our department, but also the whole organization.
Yes, I’d say the word ‘but’ probably is more likely to be omitted when the previous clause begins with ‘Not only’ (and thus has inversion). Yes, sentences 2 and 3 are also OK.
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