Learn Italian: Nessuno compra un'arancia aspra se è difficile da sbucciare. - Nobody buys a sour orange if it is hard to peel.

When an adjective like difficile (hard), facile (easy), or impossibile (impossible) describes a noun and is followed by an infinitive verb, you must connect them with the preposition da. The pattern is always [adjective] + da + [infinitive].

Since nessuno means 'nobody', why isn't there a non in this sentence to make it negative?

In Italian, when a negative word like nessuno comes before the verb, you do not use non. You only use non if the negative word comes after the verb (for example, if the sentence were structured as Non compra un'arancia nessuno).

Why does un'arancia have an apostrophe?

Arancia is a feminine noun. The feminine indefinite article is una, but when the noun starts with a vowel, the final 'a' drops and is replaced with an apostrophe. If it were a masculine noun starting with a vowel (like un amico), there would be no apostrophe because the base masculine article is just un.

How do we know 'it is hard' refers to the orange? There is no word for 'it' before è difficile.