Learn Farsi: من یک مجموعه‌ی مخفی از جواهر داشتم.

The verb dâshtan (to have) is a major exception in Farsi. When it means possession, it almost never takes the mi- prefix. Because of this, داشتم (dâshtam) is used for both the simple past (I had) and the past continuous (I was having or I used to have). The context of the sentence tells you which one it is!

Why is there a ی added to the end of مجموعه (majmū'e)?

This is the ezāfe connecting the noun to its adjective, مخفی (makhfī). Because مجموعه ends in a silent h (which makes an e sound), you cannot just add a standard ezāfe e sound. You must add a y sound to bridge the two vowels. It is written as ی and pronounced as -ye.

Why does this sentence use از (az) to mean of?

While the ezāfe is generally used to show possession or description, the preposition از (which normally means from) is often used when talking about what a collection, group, or container is made out of. Farsi literally describes it as a secret collection from jewels.

Why doesn't جواهر (javâher) have a plural ending like ها (-hâ) if it means jewels?

The word جواهر is a borrowed Arabic plural, meaning it already translates to jewels all by itself. Farsi speakers often use these borrowed plural words as collective nouns without needing to add a Persian plural ending.