Learn Italian: Nel preparare il panino, ho aggiunto del pecorino. - While preparing the sandwich, I added some pecorino.

In English, we often drop the word "some" and just say "I added pecorino." In Italian, while you can sometimes omit it, using the partitive article (di + definite article) is much more natural when referring to an unspecified quantity of an uncountable noun like pecorino. Here, del is formed by combining di + il.

Could I use un po' di instead of del in this sentence?

Yes, you could say ho aggiunto un po' di pecorino. Both mean "some pecorino," but un po' di slightly emphasizes a small amount ("a little bit of"), whereas the partitive del neutrally states an indefinite amount ("some").

Why does the sentence use Nel preparare to mean "While preparing"?

In Italian, the preposition in combined with a definite article (here, in + il = nel) followed by an infinitive verb is a very common way to express "while doing something" or "in the process of doing something."

Could I use the gerund preparando instead of nel preparare?

Absolutely. Saying Preparando il panino... means the exact same thing. Using nel + infinitive is just an alternative that can sound slightly more formal or put a bit more focus on the specific timeframe of the action.