NOT ONLY + present perfect continuous (inversion) + BUT

Adverb expressions such as ‘not only’, ‘not just’, and ‘not simply’ emphasize that something is true, but it is not the whole truth.  These co-ordinate clauses.

C2 point 92 in the category of PAST is defined:

present perfect continuous, invert the subject and affirmative auxiliary verb with ‘not only … but’ as a focusing device

EXAMPLES:

Not only has Pablo been talking publicly for the first time about being an orphan,

but together, we started a live storytelling series we’re calling Talk Rx,

and it’s become really popular place for his peers to show their most vulnerable and powerful thoughts and feelings.

TED

 

Not only have they been advancing from the east,

but they’re coming down on the northern and the southern fronts as well.

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