Learn Italian: Scusi, Lei è più giovane di Marco? - Excuse me, are you younger than Marco?

In Italian, you don't add endings like "-er" to adjectives to make a comparison. You simply use più (more) followed by the adjective. So, "younger" is always expressed literally as "more young" (più giovane).

Why is the word di used here for "than"?

When you directly compare two people, things, or pronouns (in this case, Lei and Marco) using an adjective (giovane), you use di to mean "than".

Why is Lei capitalized? Doesn't that word usually mean "she"?

When written with a capital letter, Lei is the formal, polite way to say "you" to one person. Because it is grammatically third-person, it takes the same verb form as "he" or "she", which is why the sentence uses è instead of the "you" form sei.

Why is the sentence using Scusi instead of scusa for "excuse me"?

Scusi is the formal version of "excuse me". It matches the formal, polite Lei used later in the sentence. If you were talking to a friend, you would use the informal scusa and ask tu sei più giovane di Marco?.