Through the book did

[Erbab]

''Only when he was halfway through the book did he began to be interested in the plot.''

This sentence is easy to understand. But I did not comprehend the necessity of ''did''. What did the book do?

''When he was halfway through the book, he began to be interested in the plot.'' This makes more sense.

Thanks for your help.

 

[emsr2d2]

The original has an error - it should be "begin", not "began".

"did" is not connected to "the book". It is connected to "he".

When he was halfway through the book, he began to be interested in the plot.
Only halfway through the book did he begin to be interested in the plot.