Mea interest quando navis oneraria proficiscatur.

Because interest in the sense it matters / it concerns uses a special construction in Latin.

• With nouns, Latin often uses the genitive: magistri interest = it concerns the teacher.
• With personal pronouns, classical Latin normally uses the fixed forms mea, tua, sua, nostra, vestra.

So mea interest is the standard way to say it matters to me.

For a learner, it is best to memorize mea interest as a set expression.

Here interest is a verb, not a noun.

It is the 3rd person singular present of intersum, interesse, used impersonally:

• interest = it matters
• it is important
• it makes a difference

So mea interest literally works like it matters to me.

It happens to resemble the English word interest, but grammatically in this sentence it is definitely a Latin verb.

Because quando navis oneraria proficiscatur is an indirect question.

The hidden direct question is:

• Quando navis oneraria proficiscetur?
• When will the cargo ship depart?

When Latin turns a direct question into an indirect one, it normally puts the verb into the subjunctive.

So:

• direct question: quando ... proficiscetur?
• indirect question: quando ... proficiscatur

This is a very common rule in Latin.

Because Latin does not have a future subjunctive.

In an indirect question after a present main verb such as interest, Latin often uses the present subjunctive for something that English would naturally translate as will depart.

So proficiscatur can mean:

• departs
• is departing
• will depart

The exact English wording depends on context, but in this sentence a future translation is very natural.

Because it comes from a deponent verb:

• proficiscor, proficisci, profectus sum

A deponent verb has:

• passive-looking forms
• but an active meaning

So proficiscatur looks passive in form, but it means:

• he/she/it sets out
• he/she/it departs

It does not mean is departed or is sent off.

Both words are nominative singular feminine.

• navis = ship
• oneraria = cargo-bearing / freight / merchant

They are nominative because they form the subject of proficiscatur.

Also, oneraria agrees with navis in:

• gender
• number
• case

So navis oneraria means the cargo ship.

oneraria is an adjective from the idea of load / burden / cargo.

So navis oneraria literally means something like:

• a cargo-carrying ship
• a freight ship

In smoother English, we simply say cargo ship.

Latin often uses an adjective where English prefers a noun modifier.
So the phrase is perfectly normal Latin even though the most natural English translation is not cargo-bearing ship.

Because Latin word order is much freer than English word order.

Both of these are possible:

• navis oneraria
• oneraria navis

The version in your sentence is completely normal. Latin often places adjectives either before or after the noun depending on style, emphasis, and habit.

So here you should rely more on the endings than on the word order.

Because quando is the normal question word meaning when?

That makes it the right choice for both:

• a direct question
• an indirect question

Here the clause is really asking when the ship departs, so quando fits naturally.

By contrast, cum is usually a conjunction meaning:

• when
• since
• although

It is not usually the basic interrogative when? that you use to ask for a time.

The sentence has two parts:

1. mea interest
   = it matters to me

2. quando navis oneraria proficiscatur
   = when the cargo ship departs / will depart

So the structure is:

• main clause: it matters to me
• embedded indirect question: when the cargo ship will depart

That is why the sentence combines:

• the idiom mea interest
• the interrogative word quando
• and the subjunctive proficiscatur