Cum pluvia per compluvium cadit, impluvium paulatim completur.

Here cum means when, not with.

It is a conjunction introducing a subordinate clause: Cum pluvia per compluvium cadit = when rain falls through the compluvium.

Latin cum can mean different things depending on context:

• with as a preposition: cum amico = with a friend
• when/since/although as a conjunction, as it does here

Because this sentence is presenting a straightforward fact or regular occurrence.

So cadit is present indicative: falls. That is normal when cum simply means when in a factual or general sense.

A learner may expect cum + subjunctive because that also happens in Latin, especially in narrative prose, where cum can mean things like when, since, or although with a more interpretive sense. But here the idea is plain and literal: when rain falls...

Pluvia is nominative singular, and it is the subject of cadit.

So:

• pluvia = rain
• cadit = falls

Together: rain falls

English speakers sometimes find this a little odd because rain is often treated as a mass noun, but Latin can also use pluvia as a regular singular noun.

Per takes the accusative case, so compluvium here is accusative singular.

Per compluvium means through the compluvium.

That fits the sense of motion through something:

• cadit = falls
• per compluvium = through the roof opening

So the phrase shows the path the rain takes.

These are two different parts of a Roman house.

• compluvium: the opening in the roof
• impluvium: the basin or pool below it that collects the rainwater

So the sentence describes a very specific Roman architectural setup: rain falls through the compluvium and gradually fills the impluvium.

The two words are easy to confuse, so it helps to remember their roles:

• compluvium = above
• impluvium = below

Because impluvium is a neuter noun of the second declension.

In Latin, neuter nouns have the same form in the nominative singular and accusative singular:

• nominative singular: impluvium
• accusative singular: impluvium

So even though it looks like an accusative to an English-speaking learner, here it is nominative because it is the subject of completur.

The same form appears with compluvium, but there it is accusative because it follows per.

Because the impluvium is receiving the action, not doing it.

• complet = fills
• completur = is filled

So:

• impluvium paulatim completur = the impluvium is gradually filled

This is a present passive indicative, third person singular.

The ending -tur is the key passive marker here.

Yes, often it can.

Even though the Latin form is passive, English sometimes prefers a more natural intransitive phrasing:

• is gradually filled
• gradually fills up

Both reflect the Latin well in context.

Grammatically, though, the Latin is still passive in form.

Paulatim is an adverb meaning gradually, little by little, or step by step.

It modifies completur:

• completur = is filled
• paulatim completur = is gradually filled

Its position is flexible. Latin often places adverbs where they sound natural or where the writer wants emphasis.

Because Latin has no articles.

So a noun like pluvia can mean:

• rain
• the rain
• sometimes even a rain depending on context

Likewise:

• compluvium can mean the compluvium
• impluvium can mean the impluvium

English has to choose an article when translating, but Latin usually leaves that to context.

The subject is impluvium.

That can be easy to miss because English speakers may focus on pluvia from the first clause. But each clause has its own verb and subject:

First clause:

• pluvia = subject
• cadit = verb

Second clause:

• impluvium = subject
• completur = verb

So the sentence moves from what the rain does to what happens to the basin.

Most naturally, it expresses a general or regular process.

So the sense is something like:

• when rain falls through the compluvium, the impluvium gradually fills
• or whenever rain falls through the compluvium, the impluvium fills little by little

Latin present tense often does this, especially in descriptive or general statements.

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

This sentence is arranged quite naturally:

• subordinate clause first: Cum pluvia per compluvium cadit
• main clause second: impluvium paulatim completur

Within the clauses, the order also helps the picture unfold:

1. pluvia — the rain
2. per compluvium — through the roof opening
3. cadit — falls
4. impluvium — the basin below
5. paulatim completur — gradually fills

So the order is not random; it matches the scene being described.