The following test question appeared for turning regular sentences into questions without using 'wh- words':
The dog had to be taken to the vet because he had eaten a whole shoe.
The answer should be: Did the dog have to be taken to the vet because he had eaten a whole shoe?
However, many students mistook 'had' for an auxiliary verb, and turned the sentence into: Had the dog to be taken to the vet. (Switching the auxiliary verb and the subject.)
I have been trying to think of a way to explain this to them when we come to discussing it, but I'm having trouble picking apart the grammatical construction of 'had to be taken'.
It looks like the present perfect, but then why wouldn't the auxiliary rule work, and I am not sure how to place the 'to be'.
I feel like I should know what this is, but I can't figure it out.