Learn Farsi: مسافری که ساکش گمشده است، باید آن را توصیف کند. - The passenger whose bag is lost must describe it.

Farsi doesn't have a single word for "whose." Instead, it uses the connecting word ke (that/who) followed by the noun with a possessive ending attached. Here, we see ke followed by sâk-ash (bag + his/her). Literally, the structure translates to "who his/her bag is lost."

The translation says "The passenger," but mosâferi has the -i ending that usually means "a passenger." Why?

When a noun is followed by a relative clause that specifies exactly who or what we are talking about, Farsi adds an -i to the noun. This is sometimes called the restrictive -i. So mosâferi ke translates best to "the passenger who..." rather than "a passenger who..."

Why is gom shodeh ast used here instead of a simple past tense?

This is the present perfect tense (has become lost / is lost). It is used here because the action happened in the past but the result continues into the present: the bag is still missing right now, which is why the passenger must describe it.

Why is the verb towsif konad (describe) in the subjunctive form?

The word bâyad (must) always requires the verb that follows it to be in the subjunctive mood. For the compound verb towsif kardan (to describe), the action part kardan becomes konad to match the third-person singular passenger.