How to use "used to use"?
It's fine to use that way as far as you maintain that you no longer "open the cans using a knife". Also, "used to" is a modal verb that can be used with any verb and in this case, "use."
Yes, used to use is a perfectly normal construction.
That’s fine (as per Barrie and Manoj), but you may want to say a knife, unless you always did your opening with a specific knife that’s contextually salient.
On the same theme as your question, you can create other sentences where a verb is used both in its “lexical” and its “grammaticized” senses. For instance:
- I have to have it. [modal have and lexical have]
- He has to have had it. [modal have, perfect have, lexical have]
- She had me have him over. [causative have, lexical+particle have]
- Who knows what he will will. [auxiliary will, lexical will]
- She’s going to go. [auxiliary go, lexical go]
- Who will they make make a cake. [causative make, light verb make]
If you get carried away, you could end up with Who has she had to have have you over?
Yes, used to use is perfectly normal. I normally pronounce it uste instead of uzed. Just remember that it also means you don't anymore. "Used to" is a verb and the rest of the sentence is the direct object. I believe it would be a bare infinitive.