Cum pluvia per compluvium cadit, impluvium paulatim complētur.

Here cum means when.

It introduces a temporal clause, giving the time when the main action happens:

• Cum pluvia per compluvium cadit = when rain falls through the compluvium
• impluvium paulatim complētur = the impluvium is gradually filled

A learner may already know that cum can also mean with, but that is a different use. In this sentence, cum is a conjunction, not a preposition.

Cadit is the 3rd person singular present active indicative of cadere.

It is present because the sentence describes something that happens generally or regularly:

• pluvia ... cadit = the rain falls
• not a single one-time event, but a typical process

Latin often uses the present tense for general truths or repeated actions, just as English does in sentences like when it rains, the floor gets wet.

Pluvia is nominative because it is the subject of cadit.

In the clause:

• pluvia = the thing doing the falling
• cadit = falls

So pluvia must be in the nominative case.

A native English speaker may expect something like it rains, but Latin can simply use rain itself as the subject: pluvia cadit.

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

Per usually means:

• through
• sometimes by means of
• sometimes across

So:

• per compluvium = through the compluvium

This is a very common pattern in Latin:

• per viam = through the road / along the road
• per portam = through the gate
• per fenestram = through the window

These are two related but different architectural terms from a Roman house.

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

A helpful way to remember them is by their prefixes:

• com-/con- can suggest things coming together
• im- here is related to the idea of rainwater flowing into something below

In the sentence, the rain:

1. falls through the compluvium
2. fills the impluvium

So the two words are easy to confuse, but they refer to different parts of the house.

Complētur is 3rd person singular present passive indicative of complēre, meaning to fill or to fill up.

So:

• impluvium complētur = the impluvium is filled / fills up

Latin often uses the passive where English might also use a passive or a middle-like expression:

• the pool is filled
• the pool fills up

The focus is on what happens to the impluvium, not on naming an agent such as someone fills it.

Yes, often that is the most natural English translation.

Grammatically, complētur is passive, but in English we often prefer an intransitive expression:

• literal: is filled
• natural: fills up

Both reflect the same Latin form here. The passive in Latin does not always need a very stiff passive translation in English.

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

It modifies complētur:

• impluvium paulatim complētur = the impluvium gradually fills up

So it describes how the impluvium becomes full.

Latin word order is more flexible than English word order because case endings show each word’s function.

Putting the verb at the end is very common in Latin, especially in straightforward prose:

• impluvium = subject
• paulatim = adverb
• complētur = verb

So the order is perfectly normal. English usually wants The impluvium gradually fills up, but Latin can comfortably place the verb last.

Both verbs are singular because each one has a singular subject:

• pluvia cadit: pluvia is singular
• impluvium complētur: impluvium is singular

So:

• cadit = it falls
• complētur = it is filled / fills up

Latin verbs must agree with their subjects in number and person.

Yes. The ending -um strongly suggests these are neuter singular nouns of the second declension.

That helps explain their forms:

• nominative singular: impluvium
• accusative singular: compluvium

Because neuter nouns have the same form in the nominative and accusative singular, a learner might wonder why one -um word is the subject and the other is the object of per. The answer is that their roles are shown by syntax and by the preposition:

• per compluvium: accusative after per
• impluvium complētur: nominative subject of the passive verb

The comma separates the cum clause from the main clause:

• Cum pluvia per compluvium cadit, ...
• impluvium paulatim complētur.

This is similar to English punctuation with an introductory clause:

• When rain falls through the opening, the basin gradually fills.

Latin manuscripts originally did not always use punctuation the way modern editions do, but in printed Latin a comma here is very normal and helpful.

The macron marks a long vowel:

• complētur has a long ē

Macrons are usually added in textbooks and learning materials to show vowel length. They are very useful because vowel length can affect:

• pronunciation
• rhythm
• sometimes the distinction between forms

In many ordinary printed Latin texts, macrons are omitted, and you would simply see completur.