we note the presence of the preposition "di" introducing a prepositional phrase traditionally labelled “complemento di specificazione.” The first instance, however, is an expression of place (a locative, if you like technical terms):
1a) The Trade Fair that takes place in Milan;
the second is an example of possessive case:
2a) The book that belongs to John (certainly NOT *The book that takes place in John); See
in less frequent cases, “di Giovanni” may mean "about John", if Giovanni is the topic of the book; or, finally, “di Giovanni” may mean "by John" if he is the author.
An analysis of the third example is more complex and if “complemento di specificazione” is too generic, we can label it as “complemento di abbigliamento” (something like "adverbial of dress") — which shows that such labels are arbitrary and not always useful.
Other phrases with "di" are related to notions of time,
4) Viaggiare di sabato
subject or topic,
5) Parlare di grammatica
6) Discutere di politica
or cause
7) Tremare di paura;
then there are comparisons,
8) Meglio di ieri
"di" before an infinitive,
9) Sono lieto di saperlo
the use of "articled forms" in partitives
10) Dammi del pane
and several more cases, including:
11) Coperto di neve
12) Una commedia di G.B. Shaw.
Comparative analyses are the most immediate test to highlight differences; the Eng. translation of the examples gives
1’) The Milan Trade Fair
where the location becomes a premodifier;
2’) John’s book
where the owner takes the mark of the possessive case;
3’) Dressed in grey
4’) Travelling on Saturday(s)
5’) Talking about grammar
6’) Discussing politics
7’) To tremble with fear
8’) Better than yesterday
9’) I’m glad to hear that
10’) Give me some bread
11’) Covered with snow
12’) A play by G.B. Shaw
where we find that It. "di" is translated with the Eng. prepositions "in, on, about, with, by" the conjunction "than", the partitive "some", "to" before an infinitive and even ø (zero) as in (6’). A lot more examples could be added, including those where It. "di" corresponds to Eng. "of" and are regarded as following the “general rule”:
13) La metà di giugno
13’) The middle of June
In short, "doing grammar" always implies that meanings are taken into account.
As a consequence the often has a crucial role: in some cases