Question about 'Used to'

[TomCT]

Quick question, using the expression 'used to' as in 'I used to live in London'.

Is used a verb, and is it the past participle of 'to do' (infinitive)

Any help is greatly appreciated :)

[Jo]

"Used" is a verb, the past tense of the verb "use".

Think about it this way:
Did you use to live in London?

The infinitive is "to live".

This page on used to do may help.