auxiliary verbs - Tense form after the word did

I read this explanation about did from http://www.learnersdictionary.com/qa/did-main-verb-base-form-or-past-tense-form
. Does that mean all the verb after did in a question will remain as it is?

Example like :
What did your sister use to dig the sand?

We have two verbs here and we don't have to change them to tense word?

Answer

You are largely correct. The verb 'do' is an auxiliary/helping verb. Any verb that comes after it remains in the infinitive form, meaning that it is unconjugated.

What did your sister use to dig the sand?

Here, the verb 'did' is the conjugated auxiliary verb, so you need not conjugate 'use' any further. However, the reason you do not conjugate 'dig' here is because it is part of the to-infinitive.