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Will someone please help me, I am a teaching trainee and I am stuck on this question.
What is the tense / form of 'used to have?' As in 'I used to have a dog'.
Is it the past of 'to use' + the infinitive of 'to have',
a sort of 'past progressive of to have'
or is it 'used to + have?'
Yes, it's "used to + have", "used to" indicates a past habit or situation which no longer pertains; so you can have "used to plus verb" to indicate anything in that category.
Syntactically "used to' is the structural element of a compound verbal predicate. It can't be treated as a finite form of 'use'. I believe it is either a phasal or a modal unit.
'Used to have' 
Linear Mode
