Praefectus portus nautas monet ne nocte exeant, si ventus iterum crescat.

Latin normally does not use articles the way English does. So:

• praefectus can mean the prefect or a prefect
• portus can mean of the harbor/port
• nautas can mean the sailors
• ventus can mean the wind

Context tells you which English article makes best sense.

Here portus is most naturally genitive singular, meaning of the harbor/port, depending on praefectus.

It looks the same as the nominative because portus is a 4th-declension noun:

• nominative singular: portus
• genitive singular: portus

So the form is identical, but the function is different. In praefectus portus, the sense is prefect of the harbor.

Because nautas is the direct object of monet.

The pattern is:

• aliquem monere = to warn someone
• then a clause can follow explaining what the warning is about

So:

• Praefectus ... nautas monet = The prefect warns the sailors

Here nautas is accusative plural from nauta.

Monet is the 3rd person singular present active indicative of moneo: he warns.

With moneo, Latin often uses:

• a person in the accusative
• plus an ut/ne clause

So this sentence has:

• nautas = the people being warned
• ne nocte exeant = what they are being warned not to do

That is a very common construction in Latin prose.

Because after verbs like moneo, Latin commonly uses ne + subjunctive for a negative indirect command/warning.

So:

• ut
  • subjunctive = that they should ...
• ne
  • subjunctive = that they should not ...

In this sentence:

• ne nocte exeant = that they should not go out at night

English often uses an infinitive or a different structure, but Latin prefers ne with the subjunctive here.

Exeant is present subjunctive, 3rd person plural, from exeo (go out, go forth).

Breakdown:

• base verb: eo = I go
• compound: exeo = I go out
• exeant = they may/should go out

It is subjunctive because it is part of the ne clause after monet.

Nocte is the ablative singular of nox, noctis.

It is an example of the ablative of time when, which often appears without a preposition in Latin.

So:

• nocte = at night
• compare die = by day / in the daytime

English usually needs a preposition like at, but Latin often does not.

This is a very common learner question, because English speakers often expect a future indicative such as if the wind rises again.

Here crescat is present subjunctive from cresco. The subjunctive fits well because the clause refers to a possible future situation: if the wind should rise again.

Also, this si clause is part of the content of the warning, so it is natural for it to stay within the subjunctive atmosphere of the indirect command.

So the idea is not a plain statement of fact, but a contingent future possibility.

Iterum means again. It modifies crescat:

• ventus iterum crescat = the wind should rise again

So the meaning is that the wind has risen before, or might do so another time.

Latin word order is more flexible than English word order because the endings show each word’s role.

This sentence is arranged quite naturally:

• Praefectus portus = the harbor prefect
• nautas = the sailors
• monet = warns
• ne nocte exeant = not to go out at night
• si ventus iterum crescat = if the wind should rise again

English depends heavily on position for meaning, but Latin can move words around more freely for emphasis or style. The endings do most of the grammatical work.