How do I explain the grammatical construction of 'The dog had to be taken to the vet'

Question

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.

Answer 1

Auxiliary have partners with the past participle of the verb, not with the infinitive.

To be taken is the passive infinitive.

To take is the normal (active) infinitive.

My car had to be taken to the shop.

I had to take my car to the shop.

The subject changes but the verb have has the same meaning in the active and passive. It expresses the idea of a need or necessity, like must.

You have to get some sleep. You look really tired.

You have to drink water to stay alive.

The nature of the necessity is expressed in the infinitive clause, to get some sleep and to drink water (to stay alive).