Learn Farsi: او اشتهای زیادی داشت، پس دسر بزرگی خواست. - He had a big appetite, so he wanted a large dessert.

This is the standard rule in Farsi: when a noun is followed by an adjective connected by an Ezafe, the indefinite -i suffix attaches to the end of the adjective, not the noun. So "a large dessert" is formed as "dessert + Ezafe + large + a" (deser-e bozorgi).

Why is the word for appetite written as اشتهای (eshtehâ-ye) with a ی instead of just an Ezafe?

The base word for appetite is اشتها (eshtehâ). Because it ends in the long vowel ا (alef), we cannot just add the standard short "e" sound for the Ezafe. We have to add a "y" sound to bridge the vowels, which is written as a ی.

Why does the sentence use زیاد (ziâd) for a "big" appetite, but بزرگ (bozorg) for a "large" dessert?

In Farsi, appetite is quantified rather than measured by physical size. So instead of a physically "large" (بزرگ) appetite, you say you have "a lot of" (زیاد) appetite. For physical objects like a dessert, بزرگ is the correct choice.

Why don't the past tense verbs داشت (dâsht) and خواست (khâst) use the می (mi-) prefix here?

Verbs of state, like "to have" (داشتن) and "to want" (خواستن), usually do not take the می continuous prefix in the past tense. Even though the appetite and desire were ongoing states in the past, Farsi uses the simple past forms and .