"di" means "of" when you're talking about possession. For example, "This is Tom's video" = "Questo è il video di Tom." In Italian, whenever you're saying something like "Tom's video" you have to think of it as "the video of Tom." "di" also means "of" when you're talking about what something is made "of." For example: "the desk is made of wood" = "la scrivania è fatta di legno." "di" can also mean "from" when asking where someone is from. But only when we're involving the verb "essere." For example, "Di dove sei?" = "Where are you from?" Next is "da" which means "from" "da" means "from" in a similar way to "di" except for the fact that the verb "essere" is not involved. For example, "I received a gift from Tom" = "Ho ricevuto un regalo da Tom." "da" is also used whenever you're talking about going to someone's house. To say "I'm going to Tom's" or "I'm going to Tom's house" in Italian you'd say, "Vado da Tom." This is an example of what I refer to as "the Italian way" of saying something, because its not a literal translation. This also goes for when you're saying that you're going to the doctor's, "vado dal dottore" = "I'm going to the doctor's."