It might help to know other ways you can use the participle of "usare":
- Io ho usato il libro (I have used the book = avere + participle makes it present perfect)
- Il libro è usato (The book is used = essere makes it an adjective)
- Il libro è stato usato (The book has been used = still an adjective)
"Avere" is a much more common auxiliary than "essere" for the passato prossimo.
@crista_b, I think it's in there to help you learn to distinguish between the two, which is actually relevant. I would have thought it was present perfect whether I encountered it in this lesson or not, so now I know, and I'll certainly remember because I had to look through the comments to figure out why.
Sure it does---just in the case here---'e usato/a. "Usare", to use, is a transitive verb. (Uso--I use, ho usato---I used). But using essere with a transitive verb converts the verb to passive form---'e usato (is used), era usato (was being used), e stato usato (was used), era stato usato (had been used). The participle (usato) now plays the role as an adjective, but it is still the participle of the verb usare.
(I can't reply directly to FidoGracie, but this is a response to his post.) We are only speaking of the auxilliary used to contruct the passato prossimo. If you look in a dictionary, when it says that usare takes avere as auxillary, that's what it means. You are correct that essere is a different kind of auxilliary in the above sentence (used to construct the passive, as you say), and, of course, in that usage, it can only take transitive verbs.
Another example of testing a point of grammar before it has actually been introduced. Clearly we have to be psychic to hang on to our hearts. I considered "is used" because the construction is familiar from studying French but I thought this section was testing the past tense!
This is a deliberate curve ball from DL: they are testing whether you can distinguish essere used as an auxiliary for passato prossimo tense and essere used for other reasons. Here it is other reasons, because usare is not in the 'house of essere' and also not a reflexive verb. See http://i.imgur.com/GNzJY9Y.jpg and http://italian.about.com/od/verbs/a/italian-verbs-auxiliary.htm.
You will see many more curve balls ahead, just as you would in real life Italian, so be prepared. My weakness is forgetting to use essere as auxiliary with reflexive verbs. ;-)
This is a use of the past participle as an adverb. If you were to use the present tense, you would say, "Which room do I use (you use, he use) as a kitchen?" "Questa stanza uso come cucina." Using the present indicative of essere and the past participle provides us with the PRESENT perfect tense but in this instance, the past participle is now used as an adverb.
Other examples that you will see are, "Mio fratello è morto." My brother is dead (due to the fact that he has died). "Mio fratello muore." My brother dies.
Comments for correctness are welcome.