When to you use "che" and "di" for comparison?
Ciao,

I learned from Pimsleur that you use "che" and "di" to mean "than" for comparison, depending on the objects used for comparison. It seems to say that when the number is used, "di" is used; but for all others, "che" should be used.

e.g. La benzina costa meno di 2 euro. (The gas costs less than 2 euros).
e.g. Si guida piu' velocemente in Italia che negli Stati Uniti.

But another book has the following sentence:
Perche' l'appartamento di Antonio e' piu' grande dell'appartamento di Christine?

Why isn't this "e' piu' grande che l'appartament..."? Please explain in English as my Italian is not so good.

Grazie mille!

- Yan

Question

When to you use "che" and "di" for comparison?
Ciao,

I learned from Pimsleur that you use "che" and "di" to mean "than" for comparison, depending on the objects used for comparison. It seems to say that when the number is used, "di" is used; but for all others, "che" should be used.

e.g. La benzina costa meno di 2 euro. (The gas costs less than 2 euros).
e.g. Si guida piu' velocemente in Italia che negli Stati Uniti.

But another book has the following sentence:
Perche' l'appartamento di Antonio e' piu' grande dell'appartamento di Christine?

Why isn't this "e' piu' grande che l'appartament..."? Please explain in English as my Italian is not so good.

Grazie mille!

- Yan

Answer

Ciao! see here http://www.uvm.edu/~cmazzoni/3grammatica/grammatica/comparatives.html
comparatives and superlatives (i comparativi e i superlativi)
I.  IL COMPARATIVO:
There are three comparatives:  di maggioranza (majority), di minoranza (minority), di uguaglianza (equality)
comparativo di maggioranza
più...di
più...che

comparativo di minoranza
meno...di
meno...che
you use "DI":
1.when two terms are compared with respect to one quality/action
2.in front of numbers

you use "CHE"
3.when there is one term and two qualities/actions refer to this one term
4.in front of a preposition
5.in front of an infinitive

comparativo di uguaglianza 
a. (così)...come or (tanto)...quanto 
(for adjective and adverbs:  "così...come" and "tanto...quanto" are adverbs and there is no agreement)
b. (tanto)...quanto
(for nouns:  here "tanto...quanto" are adjectives and there is agreement)
c. (tanto) quanto... 
(for verbs:  "(tanto)...quanto" are adverbs and there is no agreement)

così and tanto are optional and usually avoided

II. IL SUPERLATIVO RELATIVO
This is one of the easiest grammatical points in Italian ("uno dei punti grammaticali più facili dell'italiano").  The relative superlative is formed by:
the definite article (il, la, i , le) + (noun) + più/meno + adjective + di + the term in relation to which we are comparing
(it is like English, really, except that in Italian you use "di" instead of "in")