In Classical Greek there is a distinction between the subjunctive and the optative, but in Latin the former is used for the latter, e.g. "et ne nos inducas in tentationem" ('and lead us not into temptation', lit. 'and may you not lead us into temptation'). In French, one says "Prière de ne pas fumer." Et ne nos inducas in tentationem
Jeffrey, "would you come" (present conditional as you correctly say) and "come!" mean two different things. The first is a polite request, the second is a command granted, softened in tone by the per favore. As Coloraday has said, 'venga' is an imperative form based on the subjunctive.
For a god description of the subjunctive tenses read http://www.italianlanguageguide.com/grammar/subjunctive/ and http://italian.about.com/od/verbs/a/italian-verbs-present-subjunctive-tense.htm
I did read the ethim are TRA from Latin INTRA and FRA from INFRA but I have understood usually the under-meaning is for "come at/on the 3rd day": maybe we Italian are procrastinators and nobody come before the last day so the remaining meaning is this. For WHITHIN there is ENTRO: but who would risk a bother before the most far moment possible?:-D Just checked on the Garzanti dictionary: correct as I said! Otherwise TRA=FRA mean(s) BETWEEN and AMONG. I prefer TRA for BETWEEN and FRA for AMONG, in my language. Surely is not worthy to mind the euphony: it is personal matter of tastes! in poetry, allitteration can be fine, so why not poetry everywhere?
Here are some examples of the usage of tra tre giorni which seems to mean "in three days" i.e. literally three days rather than less than that.
And here are some examples of entro tre giorni which seems to mean "within three days". Even in the examples where it is translated as "in three days" these seem to be in situations where we use "in" when we actually mean "within" in English
Vito: Italian, like German which by the way I see you're also studying, has both informal and formal imperatives: informal used with friends, family members, children, pets in short anyone with whom you're on a first name basis; formal imperatives are used with adult strangers, "superiors" at work e.g., in short anyone whom you'd address with a title: Mr. Mrs. Dr. Sir, etc. In Italian as I'm sure you're aware, there are 2 informal imperatives: "you" singular and plural (tu/voi); as well as the formal imperatives.
Sorry, I just studied in Italian classical high school and I like Italian language and classics, I am not qualified for foreign Italian grammar books for foreigners. But I well remember that an English grammar book for Italian high classic school named English verb modes and tenses in its own special names. Language is art not only science. If you feel fine in thinking at Italian imperative as shared in 2 informal and one formal kinds, and your speeches are understood by whom you like, it is OK. According to me, there are just formal and informal ways to address any kind of phrases to other persons: 2nd singular person TU may be substituted by 3rd singular person LEI or (in some areas now and mostly some decades ago) by 2nd plural person VOI, as English people have chosen to do even for partners; and such way seems to be more respectful for most people. The same mechanism is for 2nd plural VOI that may be shifted into the 3rd plural LORO, but thanks to God this is unusual, and via internet the 2nd singular simple TU is gaining ground too. Bye!
Vito: That's all well and good, but the forms you use for statements or questions and those you use to tell someone to do something (imperative) can be different. You eat = (tu) mangi. Eat! = mangia! I sure you see there are other examples of differences in verb forms for statements and those for commands.
Yes, indicative and imperative are 2 different modes for verbs conjugation. In Italian I feel just one present imperative (a future imperative ,if liked biblical style, is formally equal to future indicative).
It can be hardened and softened and given gently by surroundings: threatens, "absolutely", "please" ("per favore" o "per cortesia" in Italian), smiles etc..
Other ways like could you = POTRESTI, I wonder if you would be so kind to, you cannot do this -- could you?, etc. are not imperative.
In Italian there are three words which are really interchangeable. tra/fra ; nulla/niente ; anche/pure.
In this case you could also use: Per favore venga fra tre giorni. Sometimes the decision which word is to use based only on euphony. (i.e. I learned that it will be better not to use the preposition "tra" if the following word starts with a "t", but how you can see also per DL they are interchangeable)
If you write "per favore" at the start of the sentence or at the end it tecnically wont make much difference. When u start the sentence right away with a formal imperative it might Sound much as an Order, something that somebody with higher role then you might tell, as a teacher does with young students or as a police officer might do. Starting with "per favore" makes it Clear right away that it s not an order, it makes you understand it s a polite and chill request. This is the only real difference i would feel.