Caementa fundamentum firmum reddunt.

Caementa is the subject.

A learner might hesitate because -a often looks singular at first, but here caementa is a neuter plural form. You can also confirm it from the verb:

• reddunt = they make / they render
• so the subject must be plural

That means the sentence is structured like:

• Caementa = the stones / masonry / rubble
• fundamentum = the foundation
• firmum = firm
• reddunt = make

So: Caementa ... reddunt = The stones/masonry make ...

Because caementa is a second-declension neuter plural noun.

In Latin, many neuter nouns work like this:

• singular nominative/accusative: -um
• plural nominative/accusative: -a

For example:

• templum = temple
• templa = temples

So:

• caementum = a piece of stone, rubble-stone, quarry stone
• caementa = stones, rubble, masonry materials

This is one of the first places where English speakers often need to adjust: -a does not always mean feminine singular in Latin.

We know from both case and verb agreement.

• fundamentum can be either nominative singular or accusative singular
• but the verb is reddunt = they make, which is plural
• so the subject cannot be singular fundamentum
• therefore caementa must be the subject, and fundamentum is the direct object

So the grammar is:

• caementa = nominative plural subject
• fundamentum = accusative singular direct object

Firmum is describing the object fundamentum after the verb reddunt.

This is a very common Latin pattern with verbs meaning make, render, keep, consider, and similar ideas:

• object + adjective
• reddere aliquem/aliquid + adjective = to make someone/something + adjective

So here:

• fundamentum = the foundation
• firmum = firm

Together:

• fundamentum firmum reddunt = they make the foundation firm

This is sometimes called a predicate adjective or object complement.

Because Latin is using an adjective, not an adverb.

In English, after make, we also usually use an adjective:

• make the foundation firm
• not make the foundation firmly

Latin does the same thing:

• firmum = firm (adjective)
• it agrees with fundamentum

Since fundamentum is:

• neuter
• singular
• accusative

the adjective must also be:

• neuter
• singular
• accusative

So we get firmum.

Yes—reddo can mean give back, but it also commonly means make, render, or cause to become in the right construction.

That is what is happening here.

A useful pattern to remember is:

• reddere + object + adjective
• = to make/render + object + adjective

So:

• fundamentum firmum reddunt
• literally something like they render the foundation firm
• natural English: they make the foundation firm

This is a very normal Latin use of reddo.

Because Latin word order is much more flexible than English word order.

English relies heavily on position:

• The stones make the foundation firm

Latin relies more on endings:

• caementa tells you the subject
• fundamentum tells you the object
• firmum agrees with fundamentum
• reddunt gives the action and confirms the subject is plural

So Latin can place words in an order that feels more natural or emphatic in Latin style. Here, the verb comes at the end, which is very common:

• Caementa fundamentum firmum reddunt.

Other orders could also be possible in Latin, though they may sound slightly different in emphasis.

Because Classical Latin has no definite article and no indefinite article.

So Latin does not usually have separate words for:

• the
• a / an

Whether you translate a noun as the foundation, a foundation, or just foundation depends on context.

So:

• fundamentum could mean the foundation or a foundation
• caementa could mean the stones, stones, or masonry materials

English has to choose; Latin often leaves it unstated.

Not exactly, and this is a very common vocabulary trap.

Caementa in Latin usually means things like:

• quarry stones
• stone chips
• rubble
• masonry material

It does not mean modern cement in the usual sense of a powdered binding material.

The English word cement is historically related, but the meanings are not identical. So when reading Latin, it is safer to think of caementa as stones/rubble/masonry rather than modern cement.

Not quite.

• fundamentum is the direct object
• firmum is a complement describing that object

So the sentence does not mean The stones make the foundation and something firm. Instead, it means:

• The stones make the foundation firm

A good English comparison is:

• They painted the door red.
  • the door = object
  • red = complement describing the object

Latin is doing the same kind of thing here.

No, not in this sentence.

Even though fundamentum could theoretically be nominative singular in form, the verb prevents that reading:

• fundamentum = singular
• but reddunt = plural (they make)

If fundamentum were the subject, the verb would need to be reddit (it makes), not reddunt.

So the only grammatical reading here is that caementa is the subject.

Reddunt is:

• 3rd person plural
• present tense
• active voice
• indicative mood

from the verb reddo, reddere, reddidi, redditum.

So it means:

• they give back
• or, in this context, they make / render

The ending -unt is the normal present active ending for 3rd person plural in many third-conjugation verbs like reddere.

They are all forms that a learner will often meet early on.

• caementa comes from caementum, a second-declension neuter noun
• fundamentum is also a second-declension neuter noun
• firmum comes from firmus, firma, firmum, a first/second-declension adjective

That is why fundamentum and firmum match so neatly:

• both are neuter singular accusative
• both therefore end in -um

This agreement is one of the key things Latin learners need to watch for.