Learn Italian: Oggi il cielo è nero e quella stella è bella. - Today the sky is black and that star is beautiful.

Quello (that) acts a lot like the word "the" in Italian (il/la/l')—it changes depending on the noun that follows it. Since stella (star) is a feminine noun that would use la (as in la stella), quello changes to its matching feminine form: quella.

Why does the first è have an accent mark, but the e in the middle doesn't?

The accent mark completely changes the word's meaning! È (with the accent) means "is" (from the verb "to be"). E (without the accent) simply means "and".

Why does nero end in -o while bella ends in -a?

Adjectives must match the gender of the noun they describe. Cielo (sky) is masculine, so it takes the masculine ending -o (nero). Stella (star) is feminine, so its adjective needs the feminine ending -a (bella).

How do you pronounce cielo?

In Italian, the combination ci followed by a vowel makes a "ch" sound, like in the English word "cheese". So cielo is pronounced roughly like "cheh-lo", not with an "s" sound.