Learn Italian: Questa pista è difficile, ma quella del principiante è facile. - This slope is difficult, but the beginner's is easy.

Quella translates to "that" or "that one," and here it replaces the feminine singular noun pista to avoid repeating it. While English says "the beginner's", Italian phrasing is literally "that of the beginner" (quella del principiante).

Why is it del principiante instead of just di principiante?

In Italian, you almost always need the definite article before a noun, even after di (of). Since "the beginner" is il principiante, you combine di + il to get the articulated preposition del.

The word principiante ends in -e. How do we know it is masculine here?

We know it is masculine because of the combined preposition del (di + il). Principiante is a noun that has the same form for both genders. If the beginner were a woman, you would use la, making the phrase della principiante (di + la).

Can pista mean things other than a ski slope?