One can also look at Word Reference if there is confusion: wordreference.com In this instance their explanation of the reflexive verb "tenersi" UNDER THE COLLINS (dictionary) TAB shows that tenersi means "to hold onto." http://www.wordreference.com/iten/tenere and reflexive verbs always take "essere" as the auxiliary.
I'm not an expert of English language, but I think that "to himself" is redundant, and it's good if you want to highligth that the professor uses the key only for himself and doesn't share it. If it is so the Italian translation would be "il professore si è tenuto la chiave PER SE".
In the informal language "tenere" and "tenersi" are sinonimous and they mean only "keep": "il professore ha tenuto la chiave" = "il professore si è tenuto la chiave" different forms, the same meaning
The professor (has) kept the key to himself ... sounds better ... probably because he keeps the exam papers in his office
I do not understand the structure of this sentence. What is reflexive about holding a key ?
According to my references, the passato prossimo of tenere is "avere tenuto" rather than "essere tenuto." Therefore, "è tenuto" seems wrong to me. Anyone understand what's going on?
Here you have a reflexive verb form, with the object pronoun "si". In that case the auxiliary verb is "essere" rather than "avere."
All reflexive verbs take essere and all transitive verbs take avere. It's only the intransitive ones that are hard to figure out.
Actually, isn't the Italian wrong? Doesn't this have to be "Il professore si è tenuto a la chiave."
"a la chiave" would be wrong. "Tenere/tenersi" is a transitive verb so it requires a direct object. I think you are thinking of Spanish :-)
But if the verb is transitive, then the auxilliary should be avere, not essere. It seems like the sentence is wrong for one reason or the other.
Unless the "si" isn't reflexive here, but then I'm not sure what it could be.
Indeed this is not a reflexive verb. He does not "hold himself (tenersi) to something". He "keeps something to/for himself".
The "si" in this sentence is a "si enclitico".
See http://www.treccani.it/enciclopedia/parole-enclitiche_%28Enciclopedia_dell%27Italiano%29/, point 4.
But point 4 says that "si enclitico" selects avere not essere. Also, there is no modal verb in this sentence, so I don't think point 4 applies. (Although it's quite interesting to learn about that rule nevertheless.) :-)
I realized that that was not the best of all explanations. This a better one for the use of verbi pronominali (point 2.3): http://www.treccani.it/enciclopedia/verbi-pronominali_(Enciclopedia_dell'Italiano)/
I can undestand your confusion: this is informal and "tenersi" = "tenere" In other context you'd be right: "tenersi a qualcosa" means "grasp at something" but it couldn't be a translation for "keep" and it'd sound strange that a professor grasps at a key...! ;)