"Do you keep a diary?"
Translation:Tieni un diario?
There seems to be a problem of nuances in English. To keep a diary could be mantanere rather than tenere. To have a diary or to keep a diary - in English - would be the same (unless you get picky because to keep a diary actually means to write in it, while you can have one that you do not write in)
Does "diary" in English mean the same as "diario" in Italian. I get the feeling that when an English (at least an American English) speaker says they keep a diary, they are most likely referring to keeping a personal journal where they log daily events; whereas, they would use "calendar" or "date book" for where they keep track of appointments. Note that the cell phone apps are always calendars not diaries.
Avete is the voi conjugation of the verb avere, "to have." This would literally mean "Do you (pl.) have a diary?" which would insinuate that these people own a diary, but not that they necessarily write in it regularly, whereas "to keep," "tenere," means to own and regularly use the diary. This might be confusing because the verb tener in Spanish means to have, but the verb tenere in Italian means to keep. (Also, it'd be a little odd if multiple people (since you said "avete" rather than "hai") kept the same diary :P)