Στην πολυκατοικία υπάρχει κανονισμός που λέει ότι μετά τις έντεκα το βράδυ πρέπει να έχει ησυχία.

Στην is the contracted form of σε + την.

• σε = in / at / to
• την = the for a feminine singular noun in the accusative

So στην πολυκατοικία means in the apartment building.

In Modern Greek, σε very often combines with the definite article:

• σε + το → στο
• σε + τη(ν) → στη / στην
• σε + τον → στον

The final -ν in στην is kept before some sounds and is very common in writing.

Πολυκατοικία usually means an apartment building or block of flats, especially a building with several separate apartments.

It is made up historically from:

• πολύ- = many
• κατοικία = dwelling / residence

So the idea is a building with many residences in it.

Greek often uses υπάρχει to mean there is / there exists.

So:

• υπάρχει κανονισμός = there is a rule / regulation
• literally, a regulation exists

This is the normal way to introduce the existence of something.
Using έχει would mean has, so it would need a different structure, for example Η πολυκατοικία έχει κανονισμό = The apartment building has a regulation.

Both are possible in Greek, but Στην πολυκατοικία υπάρχει κανονισμός sounds very natural for There is a rule in the building.

Because Greek, like English, can use a noun without the definite article when it means a rather than the.

So:

• υπάρχει κανονισμός = there is a rule / regulation
• υπάρχει ο κανονισμός would mean something more like the regulation exists, referring to a specific known regulation

Here the sentence is introducing the existence of a rule, so no definite article is needed.

Που λέει literally means that says.

• που = a very common relative word meaning who / which / that
• λέει = says

So:

• κανονισμός που λέει ότι...
• a rule that says that...

Greek που is very useful because it does not change form the way English who/whom/which sometimes does. It is commonly used for relative clauses.

Ότι introduces a clause and means that.

So:

• λέει ότι... = it says that...

In English, we often drop that:

• The rule says after eleven...

Greek can sometimes omit ότι in informal speech, but using it is very normal and clear, especially in standard written language.

So the structure is:

• κανονισμός
• που λέει
• ότι μετά τις έντεκα...

= a rule that says that after eleven...

When telling time in Greek, the hour often appears with the article in the accusative after prepositions such as μετά.

So:

• μετά τις έντεκα = after eleven

Here:

• μετά = after
• τις έντεκα = literally the eleven, used in time expressions

Compare:

• στις έντεκα = at eleven
• μετά τις έντεκα = after eleven
• πριν τις έντεκα = before eleven

This use of the article with clock times is very normal in Greek.

Yes, έντεκα already means eleven, but το βράδυ specifies that it is at night / in the evening.

So:

• μετά τις έντεκα = after eleven
• μετά τις έντεκα το βράδυ = after eleven at night

Greek often adds expressions like these to avoid ambiguity:

• το πρωί = in the morning
• το μεσημέρι = at noon / midday
• το απόγευμα = in the afternoon
• το βράδυ = in the evening / at night

Yes, literally it looks like must have quiet, but this is a very natural Greek impersonal expression.

• πρέπει = it is necessary / must
• να έχει ησυχία = for there to be quiet / for it to be quiet

So the whole phrase means:

• there must be quiet
• it must be quiet
• there should be silence

Greek often uses έχει in expressions about conditions in a place:

• έχει ζέστη = it is hot
• έχει κρύο = it is cold
• έχει φασαρία = it is noisy / there is noise
• έχει ησυχία = it is quiet / there is quiet

So this is idiomatic Greek, not a word-for-word match with English.

Because ησυχία here is being used as an abstract, general idea: quiet / silence.

So:

• να έχει ησυχία = for there to be quiet

Using the article would make it more specific and less natural in this context. Greek often leaves out the article with abstract or mass nouns when speaking generally.

Compare:

• Έχει ησυχία εδώ. = It’s quiet here.
• Η ησυχία είναι σημαντική. = Quiet / silence is important.

It actually does agree with κανονισμός.

• κανονισμός is singular
• λέει is third person singular: he/she/it says

Since κανονισμός is grammatically singular, Greek uses singular λέει:

• κανονισμός που λέει... = a regulation that says...

If the noun were plural, the verb would be plural too:

• κανονισμοί που λένε... = regulations that say...

Yes. Greek word order is more flexible than English word order, although not completely free.

The given sentence is very natural:

• Στην πολυκατοικία υπάρχει κανονισμός που λέει ότι...

But you could also hear:

• Υπάρχει κανονισμός στην πολυκατοικία που λέει ότι...

The original order puts Στην πολυκατοικία first, which sets the scene: In the apartment building...

That is very common in Greek: a place or time phrase is often moved to the front for context.

All three can work depending on context.

• rule = the most general and learner-friendly translation
• regulation = often a bit more formal and probably the best fit here
• bylaw = possible in some housing or building-management contexts, but more specific

In a sentence about an apartment building, κανονισμός often refers to the building’s official rules or regulations, such as quiet hours, use of shared spaces, pets, and so on.

Yes:

• Στην πολυκατοικία = In the apartment building
• υπάρχει κανονισμός = there is a regulation
• που λέει = that says
• ότι = that
• μετά τις έντεκα το βράδυ = after eleven at night
• πρέπει να έχει ησυχία = there must be quiet

So the structure is:

In the apartment building, there is a regulation that says that after eleven at night there must be quiet.

A smoother English version would be:

There is a rule in the apartment building that says it must be quiet after eleven at night.