"È importante che si usi lo stesso calendario."
The given translation into English seems to be in the present tense "It is important to use the same calendar" This seems confusing because we are talking of subjunctives. My English grammar sometimes seems to me to be as dodgy as my Italian but how about "It is important that the same calendar be used" ? This translation would at least give me a feel that the subjunctive would be needed (by the presence of "that" = "che") otherwise I would translate this straight back into the Italian present tense. Ohh I have lost so many hearts this week!! Please help!!
Good point about che/that! That translation now works as well :)
Subjunctive is particularly tricky. Use of the subjunctive differs language to language, it almost never translates one to one! Furthermore, it is particularly rare that an English speaker understand and use it. Usually the first time you hear about the concept is in a foreign language classroom.
It might make you feel better to know that sometimes the subjunctive in Italian can be replaced with the simple present :) Teachers might not support this, but it is common.