Aquaeductus aquam ad urbem fert.

Because -us does not always mean the same thing in Latin.

Here aquaeductus is the nominative singular form, so it is the subject: the thing doing the action.

A beginner might expect a noun in -us to be a regular second-declension noun, but aquaeductus is actually a fourth-declension noun. In the fourth declension, the nominative singular often ends in -us.

So in this sentence:

• aquaeductus = the aqueduct, as subject
• fert = carries

Together: the aqueduct carries.

Because aquam is a different case of the noun aqua.

The basic form is:

• aqua = water

But in this sentence, water is the direct object, the thing being carried, so Latin puts it in the accusative case:

• aquam = water, as direct object

So:

• aqua = water
• aquam = water being acted on

This is one of the most important ideas in Latin: nouns change form depending on their role in the sentence.

You know mainly from the endings, not from the word order.

• aquaeductus is nominative singular, which usually marks the subject
• aquam is accusative singular, which usually marks the direct object

So even if the words were rearranged, the endings would still tell you their jobs.

For example, these would mean the same thing:

• Aquaeductus aquam ad urbem fert
• Aquam aquaeductus ad urbem fert
• Ad urbem aquam aquaeductus fert

Latin word order is much freer than English word order because the endings carry so much information.

Because the preposition ad takes the accusative case.

The noun’s dictionary form is:

• urbs = city

But after ad meaning to or toward, Latin uses the accusative:

• urbem = city, in the accusative

So:

• ad urbem = to the city / toward the city

This is a very common Latin pattern:

• ad + accusative

Here ad means to or toward.

It shows direction:

• ad urbem = to the city

So the sentence is describing water being carried in the direction of the city.

In many beginner sentences, ad is one of the most common prepositions for movement toward a place.

Fert means he/she/it carries or brings.

It comes from the verb:

• fero, ferre = to carry, bear, bring

This verb is irregular, so its forms do not always look like the very regular patterns you may have learned first.

Here:

• fert = he/she/it carries

The -t ending tells you the subject is third-person singular:

• he carries
• she carries
• it carries

Since aquaeductus is a thing, in English we translate it as it carries or simply the aqueduct carries.

Because Latin has no articles.

English uses:

• the
• a
• an

Latin does not have separate words for these. So aquaeductus can mean:

• the aqueduct
• an aqueduct
• just aqueduct

And aquam can mean:

• the water
• water

You decide from the context and the natural English translation.

Yes.

Latin word order is flexible because the case endings identify what each word is doing.

So these are all basically the same in meaning:

• Aquaeductus aquam ad urbem fert
• Aquaeductus ad urbem aquam fert
• Aquam ad urbem aquaeductus fert

However, different word orders can give slightly different emphasis. A writer might put an important word earlier or later for style or focus.

But the basic grammar remains the same because:

• aquaeductus is still nominative
• aquam is still accusative
• urbem is still accusative after ad
• fert is still the verb

Here it is singular.

You can tell because:

• the verb is fert, which is third-person singular
• the form aquaeductus here is nominative singular

This can be confusing because in some Latin nouns, a form ending in -us looks singular, and in others a similar-looking form can be plural. But in this sentence, everything points to one aqueduct.

Because they belong to different declensions.

Both words are accusative singular, but different kinds of nouns form the accusative differently.

• aqua, aquae is a first-declension noun

  • accusative singular: aquam

• urbs, urbis is a third-declension noun

  • accusative singular: urbem

So the case is the same, but the ending differs because the noun type differs.

This is very normal in Latin:

• same case
• different declension
• different ending

Because of the verb ending -t.

In Latin present tense:

• -o often means I
• -s often means you singular
• -t means he/she/it

So fert means:

• he carries
• she carries
• it carries

Since the subject is aquaeductus, English uses it carries or the aqueduct carries.

If the verb meant carry as a command or some other form, it would look different.

It mainly goes with the verb.

It tells you where the water is being carried:

• fert = carries
• ad urbem = to the city

So it functions like an adverbial phrase of direction:

• the aqueduct carries water to the city

It is not really describing aquam as if it were water to the city by itself; rather, it completes the idea of the action.

A learner often wants to know how to look up each form in a dictionary. The dictionary forms are:

• aquaeductus, -us = aqueduct
• aqua, -ae = water
• ad = to, toward
• urbs, urbis = city
• fero, ferre = carry, bear, bring

In the sentence, some of those forms have changed because of case or conjugation:

• aquaeductus stays in its nominative singular form
• aquam is from aqua
• urbem is from urbs
• fert is from fero

This is why memorizing dictionary forms is so important in Latin.