Learn Farsi: مسافری که چمدانش گمشده است، منتظر است. - The passenger whose suitcase is lost is waiting.

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 to it. So, mosâferi ke chamedânash literally translates to "the passenger that his/her suitcase...".

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

This -i is acting as a relative marker. When a noun is followed by a relative clause that defines exactly who or what we are talking about, Farsi often adds an -i to the noun right before ke. Even though it uses this ending, it translates to "the" in English because we are pinpointing a specific passenger.

Why is "is lost" translated as gom shode ast? Isn't that the present perfect tense?

You are right, it is formed like the present perfect, literally meaning "has become lost". However, in Farsi, using the past participle gom shode (lost) combined with ast (is) is the standard way to describe the current state of something being lost, acting much like an adjective.

Why isn't "is waiting" translated with a continuous verb tense?

Farsi often uses adjectives describing a state instead of continuous action verbs for certain concepts. Montazer is an adjective meaning "expectant" or "waiting". Combining it with ast (is) is the most natural and common way to say someone is currently waiting.