Apa se infiltra pe când instalatorul verifica tavanul.

Apă is the basic dictionary form, meaning water.
Apa means the water.

Romanian usually puts the definite article at the end of the noun, so -a here is the feminine singular definite article.

In this sentence, apa refers to specific water already understood from the situation, probably the leaking water. English might simply say water was seeping in, but Romanian often uses the definite form when the thing is specific in context.

Because the verb here is a se infiltra, which means to seep in, to infiltrate, or to leak through.

The se is part of the verb in this meaning. It does not mean the water is literally doing something to itself in the English sense. It is just how this verb is commonly used in Romanian.

There is also a non-reflexive verb a infiltra, but that usually means to infiltrate something/someone in a transitive sense, for example in military or political contexts.

So:

• a se infiltra = to seep in / to leak through
• a infiltra = to infiltrate something

Both are in the imperfect tense:

• se infiltra = was seeping in
• verifica = was checking

The Romanian imperfect is often used for:

• ongoing actions in the past
• background description
• simultaneous past actions
• repeated or habitual past actions

Here, both actions were in progress at the same time, so the imperfect is very natural.

Because the sentence describes two actions happening at the same time in the past, not two single completed events.

Compare the idea:

• Apa se infiltra = the water was seeping in
• instalatorul verifica = the plumber was checking

This gives a scene in progress.

If you changed them to completed forms, the meaning would shift. It would sound more like finished actions rather than an ongoing situation.

Pe când means while, when, or at the time when, depending on context.

In this sentence, it introduces another action that was happening at the same time:

• Apa se infiltra pe când instalatorul verifica tavanul.
• The water was seeping in while the plumber was checking the ceiling.

It can also sometimes have a contrastive meaning, like whereas, but here it clearly means while.

No, not really.

In pe când, the whole expression functions as a fixed conjunction meaning while / when. You should learn pe când as a chunk.

So here, pe is not being used in the literal spatial sense that learners often first associate with it.

Yes. În timp ce would also work very well.

For example:

• Apa se infiltra în timp ce instalatorul verifica tavanul.

That also means The water was seeping in while the plumber was checking the ceiling.

Very roughly:

• în timp ce often feels very straightforward for while
• pe când can sound slightly more literary or stylistic in some contexts, though it is still perfectly understandable and natural

Because both nouns are definite singular masculine/neuter forms:

• instalator = plumber

• instalatorul = the plumber

• tavan = ceiling
• tavanul = the ceiling

Romanian attaches the definite article to the end of the noun instead of using a separate word like English the.

So:

• -ul is the definite article here
• it means the sentence is talking about a specific plumber and a specific ceiling

Because a verifica takes a direct object.

So Romanian says:

• a verifica ceva = to check something

That is why you get:

• instalatorul verifica tavanul = the plumber was checking the ceiling

No preposition is needed.

Yes. Romanian word order is fairly flexible.

You could also say:

• Pe când instalatorul verifica tavanul, apa se infiltra.

This means the same thing, but the focus changes slightly. Starting with Pe când... puts the time frame first.

The original version starts with Apa, so it foregrounds the leaking water.

Yes, it is natural, especially if you want to suggest water penetrating through a surface, such as a wall or ceiling.

It sounds a bit more descriptive or technical than some simpler alternatives.

Depending on context, Romanian might also use:

• se scurgea = was flowing/leaking
• se strecura = was slipping through
• picura = was dripping

But se infiltra is a very good choice when the idea is that water is making its way through material.

If you used forms like s-a infiltrat or a verificat, the meaning would shift toward completed events.

For example:

• Apa s-a infiltrat... = the water seeped in / has seeped in
• instalatorul a verificat... = the plumber checked / has checked

That is different from the original, which paints an ongoing scene in the past.

So the imperfect here is important because it gives the sense of was seeping and was checking, not just seeped and checked.