Tempestas impedit quominus navis oneraria proficiscatur.

After verbs of hindering or preventing, Latin often uses quominus + subjunctive to express from doing something.

So:

Tempestas impedit quominus navis oneraria proficiscatur
means literally something like
The storm prevents that the cargo ship set out less
but idiomatically it means
The storm prevents the cargo ship from setting out.

For an English speaker, the key point is: after impedit, Latin does not normally use a preposition equivalent to English from. Instead, it often uses the conjunction quominus.

Because quominus introduces a subordinate clause after a verb of hindering, and that clause normally takes the subjunctive.

So the pattern is:

impedio + quominus + subjunctive

That is why Latin has proficiscatur, not an indicative form such as proficiscitur.

Proficiscatur is:

• 3rd person singular
• present subjunctive
• from the verb proficiscor

The verb proficiscor means set out, depart, or start a journey.

So proficiscatur means may set out, should set out, or in this construction simply set out / depart in the dependent clause.

Because proficiscor is a deponent verb.

Deponent verbs:

• have passive forms
• but active meanings

So although proficiscatur looks like a passive verb form, it means he/she/it sets out, not he/she/it is set out.

This is very common in Latin, and proficiscor is a standard example.

Because navis oneraria is the subject of the subordinate clause introduced by quominus.

In English, we say:

The storm prevents the cargo ship from setting out.

Here the cargo ship feels like the thing being prevented, so an English learner may expect an accusative in Latin. But Latin structures the idea differently:

• Tempestas = subject of impedit
• quominus navis oneraria proficiscatur = the clause being prevented
• navis oneraria = subject inside that clause

So navis is nominative because it is the one that proficiscatur.

Oneraria is an adjective meaning cargo-carrying, freight, or merchant.

It agrees with navis in:

• gender: feminine
• number: singular
• case: nominative

So:

• navis = ship
• oneraria = cargo-carrying

Together, navis oneraria means cargo ship or merchant ship.

The ending -a does not mean the noun is first declension; it simply shows the feminine nominative singular form of the adjective agreeing with navis.

Tempestas is nominative singular.

It is the subject of impedit, so it is the thing doing the preventing:

• tempestas = the storm / the weather
• impedit = prevents

So tempestas impedit means the storm prevents.

Impedit is the 3rd person singular present active indicative of impedire.

The present tense here can express:

• a present action: is preventing
• a general statement: prevents

So the sentence can be understood as either:

• The storm is preventing the cargo ship from setting out
• The storm prevents the cargo ship from setting out

depending on context.

The present subjunctive is used because the departure is viewed as happening at the same time as the main verb impedit.

In other words:

• the storm is preventing
• the ship from setting out now / at this time

This is the normal tense in a subordinate subjunctive clause when the main verb is present and the action is contemporaneous.

Yes. Latin word order is much freer than English word order because the endings show the grammatical relationships.

For example, the following would still mean essentially the same thing:

Navis oneraria quominus proficiscatur tempestas impedit.

However, the original order is clear and natural:

• Tempestas first gives the topic or cause
• impedit states the main action
• quominus navis oneraria proficiscatur explains what is being prevented

So while the order can change, the original version is a very reasonable and readable Latin sentence.

Yes, but quominus is a very standard construction after verbs of hindering like impedire.

A learner should especially remember this pattern because it comes up often:

• impedire quominus...
• deterrere quominus...
• similar verbs of preventing or hindering

So even if another phrasing might be possible in some contexts, impedit quominus ... proficiscatur is exactly the kind of construction you should expect to see in Latin.