Need help with "did"
Not until they’d stepped out into the cold night air did they throw off the cloak, glad to be able to breathe properly again.

Can someone explain му why author use "did" in the middle of that sentence?

Question

Need help with "did"
Not until they’d stepped out into the cold night air did they throw off the cloak, glad to be able to breathe properly again.

Can someone explain му why author use "did" in the middle of that sentence?

Answer

This is called an inversion after a negative adverbial. 'Not until' is a negative adverbial and when placed in front of the subject it is necessary to add an auxiliary verb in front of the subject like in questions. This kind of structure is very formal and literary and not used in normal conversation.