Cum pluvia per compluvium cadit, impluvium paulatim completur.

Here cum means when.

It introduces a time clause: Cum pluvia per compluvium cadit = when rain falls through the compluvium.

In other contexts, cum can also mean things like with, since, or although, so it is very common for learners to stop and ask which meaning is intended. In this sentence, the verb and the overall sense make it clearly temporal: when.

Because this is a straightforward temporal clause describing what happens as a regular fact.

• cadit is present indicative
• the clause means when rain falls...
• it is not a literary or more interpretive cum clause such as since or although

A useful beginner rule is:

• cum + indicative often gives a simple time idea: when
• cum + subjunctive often gives a more contextual idea such as since, although, or when in a narrative setting

So here the indicative fits the plain, factual sense.

Yes. Pluvia is nominative singular, so it is the subject of cadit.

Literally, Latin is saying something like rain falls. English often prefers it rains, but Latin can express the idea with a noun as the subject.

So:

• pluvia = rain
• cadit = falls

Together: rain falls.

Yes. Latin very commonly uses pluit, meaning it is raining or it rains.

So a learner may wonder why this sentence says pluvia ... cadit instead. The reason is probably stylistic and visual:

• pluvia cadit emphasizes the rain physically falling
• it fits nicely with per compluvium: the rain is falling through the opening
• it creates a more concrete picture than just pluit

So both are possible in Latin, but they are not exactly identical in feel.

Compluvium is accusative singular here because it follows per.

The preposition per takes the accusative and usually means through.

So:

• per = through
• compluvium = object of per
• per compluvium = through the compluvium

This is a very standard Latin pattern: preposition + its required case.

This is one of the most interesting parts of the sentence.

In a traditional Roman house:

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

So the sentence describes a real architectural process:

• rain falls through the compluvium
• the impluvium gradually fills up

The two words look similar because they are related in meaning, and that similarity can confuse learners at first.

Yes, they are related.

Both are neuter nouns, and both are built from the idea of rainwater flow:

• compluvium: the place where rain comes together or falls through
• impluvium: the place into which rain flows

You do not need to master the etymology to understand the sentence, but it helps explain why the two words look so similar and why they belong together in Roman-house vocabulary.

The subject is impluvium.

That is why completur is singular: the verb agrees with impluvium, which is a singular noun.

So the second clause works like this:

• impluvium = subject
• paulatim = adverb, gradually
• completur = is filled / fills up

Completur is the present passive of compleo, meaning fill.

So:

• complet = fills
• completur = is filled

In English, we might say either:

• the impluvium is gradually filled
• or more naturally, the impluvium gradually fills up

Latin uses the passive form because the basin is receiving the action rather than doing the filling itself.

Not here.

Even though compleo is related to the idea of making something complete, in many contexts it simply means to fill or fill up.

So impluvium completur means:

• the impluvium is filled
• or fills up

It does not mean that the impluvium is somehow finished or completed in the sense of construction.

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

It modifies completur, telling us how the impluvium is filled.

So:

• impluvium completur = the impluvium is filled
• impluvium paulatim completur = the impluvium is gradually filled

It adds the idea of a process happening over time, not instantly.

Because Latin word order is flexible, and putting the verb at the end is very common.

English depends heavily on word order for grammar, but Latin depends much more on endings. Because of that, Latin can arrange words for emphasis or style.

So impluvium paulatim completur is a perfectly normal Latin order:

• subject: impluvium
• adverb: paulatim
• verb: completur

A different order could also be possible, but this one is natural and elegant.

The present tense here expresses a general, repeated, or vivid fact.

The sentence is not necessarily about one single moment only. It can mean something like:

• Whenever rain falls through the compluvium, the impluvium gradually fills up

So the present tense works well for a regular process or general truth.

Because classical Latin has no articles like English the and a/an.

So:

• pluvia can mean rain or the rain
• compluvium can mean the compluvium
• impluvium can mean the impluvium

The context tells you how definite the noun is. In this sentence, the house features are clearly specific, so English naturally uses the in translation.

A very literal translation would be:

When rain falls through the compluvium, the impluvium is gradually filled.

A more natural English rendering might be:

When rain falls through the roof opening, the basin below gradually fills up.

Or, if you keep the Roman architectural terms:

When rain falls through the compluvium, the impluvium gradually fills up.

Yes. That is almost certainly the background.

The sentence uses two very specific architectural terms:

• compluvium: the opening in the roof
• impluvium: the basin in the atrium below

So the sentence is not just random grammar practice. It is describing how rainwater is collected in a typical Roman domestic setting. Knowing that makes the whole sentence much easier to visualize and remember.