Praefectus nautis imperat ut in portu maneant.

Because imperare works differently from English to order.

In Latin, imperare alicui means to give an order to someone, and the person receiving the order goes in the dative case, not the accusative.

So:

• nautis = to the sailors / for the sailors
• it is dative plural

That is why the sentence has:

• Praefectus nautis imperat = The prefect gives an order to the sailors

The actual content of the order is then expressed by the ut clause:

• ut in portu maneant = that they stay in the harbor

Because this is an indirect command.

After verbs of commanding, ordering, urging, advising, and so on, Latin often uses:

• ut
  • subjunctive

So:

• imperat ut maneant = he orders that they stay / he orders them to stay

The subjunctive here does not make the action doubtful. It is simply the normal grammar for this kind of clause.

Here ut introduces the clause that gives the command.

In this sentence, ut is best understood as:

• that
• or in smoother English, to

So:

• imperat ut in portu maneant = he orders that they remain in the harbor = he orders them to remain in the harbor

This is not the same as every use of ut in Latin. Depending on context, ut can also mean things like as, when, or so that. Here, though, it is the indirect command use.

The ending tells us.

maneant comes from manere = to remain, stay.

Its form is:

• present subjunctive
• active
• third person plural

The -ant ending shows they and the subjunctive vowel pattern tells you it is subjunctive.

So maneant means:

• they may stay
• they should stay
• or, in this construction, simply they stay

In natural English here: that they stay or to stay.

The subject is understood from the verb ending and the context.

• maneant = they stay
• the nearest logical they is nautis = the sailors

So the sentence means that the prefect orders the sailors to remain in the harbor.

Latin very often leaves out subject pronouns such as he, she, or they, because the verb ending already shows the person and number.

Because in takes different cases depending on whether it shows:

• location = in/on somewhere → ablative
• motion into a place → accusative

Here the sailors are not moving into the harbor; they are staying there.

So:

• in portu = in the harbor → ablative
• in portum would mean into the harbor → accusative

That is a very common Latin pattern.

Portu is ablative singular of portus.

A useful thing to notice is that portus is a fourth-declension noun. Its ablative singular ends in -u:

• nominative: portus
• genitive: portūs
• dative: portuī
• accusative: portum
• ablative: portu

So in in portu, the ablative is required because the phrase expresses location.

Praefectus is nominative singular, and it is the subject of imperat.

So:

• Praefectus = the prefect / commander
• imperat = orders / gives orders

Together:

• Praefectus imperat = The prefect orders

Because they have different subjects.

• imperat is third person singular: he/she orders
  • subject: praefectus
• maneant is third person plural: they stay
  • subject: the sailors, understood from nautis

So the sentence has:

• one person giving the order
• several people carrying it out

No, not in standard classical usage for this meaning.

With imperare, the person commanded is usually in the dative:

• nautis imperat = he gives orders to the sailors

If you want to express what they are ordered to do, Latin commonly adds:

• ut
  • subjunctive

So the structure here is exactly what you should expect:

• person ordered = dative
• content of command = ut clause

Yes, it is a very normal and important pattern.

Latin often uses indirect command after verbs such as:

• impero = order
• moneo = warn
• hortor = urge
• rogo = ask
• persuadeo = persuade

The pattern is:

• verb of ordering/asking/urging + ut (or ne for negative) + subjunctive

So:

• imperat ut maneant = he orders them to stay
• imperat ne maneant = he orders them not to stay

Because Latin often prefers a different structure from English.

English commonly uses:

• orders them to stay

Latin, however, commonly uses:

• orders to them that they stay
• that is, imperat nautis ut maneant

So even though the English translation uses an infinitive, the Latin grammar uses an ut-clause with the subjunctive.

This is a good example of why you should not try to match Latin word-for-word with English grammar.

Yes. Latin word order is more flexible than English because the endings show the grammatical relationships.

For example, the same basic meaning could appear in other orders, such as:

• Nautis praefectus imperat ut in portu maneant.
• Ut in portu maneant, praefectus nautis imperat.

The original order is perfectly natural, though:

• Praefectus first introduces the main subject
• nautis follows as the people receiving the order
• imperat gives the action
• ut in portu maneant states the order itself

So while word order can shift, it is not random; it often affects emphasis.

A very natural translation is:

• The prefect orders the sailors to remain in the harbor.

A more literal translation would be:

• The prefect gives orders to the sailors that they remain in the harbor.

Both reflect the Latin well, but the first sounds better in normal English.