Ilmoitustaululla on uusi ilmoitus: huomenna on vesikatko koko talossa.

Because Ilmoitustaululla on... is an existential/locative structure meaning There is ... on the noticeboard.

• ilmoitustaulu = noticeboard (the object)
• ilmoitustaululla = on the noticeboard (-lla/-llä = adessive case, often “on/at”)
• on
  • noun = “there is/are” in that location

If you said Ilmoitustaulu on uusi, that would mean The noticeboard is new (describing the board itself).

Ilmoitustaululla is in the adessive case (-lla/-llä). It commonly expresses:

• physical location: on something (on the board)
• more general location: at a place (at the station)
• possession-like meaning: Minulla on... = I have... (“on me there is...”)

Here it’s the concrete “on the surface of the noticeboard.”

Finnish word order is flexible, but it follows information structure. Here the sentence is basically:

• Location first: Ilmoitustaululla (setting the scene)
• Then the existential verb: on
• Then the new item being introduced: uusi ilmoitus

So Ilmoitustaululla on uusi ilmoitus is the natural “There is a new notice on the board.”
You can say Uusi ilmoitus on ilmoitustaululla, but that sounds more like you’re specifically talking about that notice and telling where it is (less like introducing it).

Because there are two separate clauses: 1) Ilmoitustaululla on uusi ilmoitus = There is a new notice on the board.
2) huomenna on vesikatko koko talossa = tomorrow there is a water outage in the whole building.

The colon connects them: the first clause announces a notice, and the second clause gives the notice’s content.

The colon introduces the content of the notice. It works like:

• A new notice: tomorrow there is...

In Finnish, a colon is very common after words like ilmoitus (notice), tiedote (announcement), huom (note), etc., when you’re about to state what it says.

Vesikatko is a compound noun:

• vesi = water
• katko = interruption/break (from the verb idea “to cut off/break”)

Finnish forms compounds very freely, so “water outage / water cut” becomes a single word. You’ll see lots of similar ones, e.g. sähkökatko (power outage).

Because the meaning is “in the whole building,” and Finnish marks that with the inessive case (-ssa/-ssä) = “in.”

• talo = house/building
• talossa = in the building
• koko talossa = in the entire building

koko talo would be “the whole building” as a subject/object, not “in the whole building.”

In contexts like building notices, talo often means the whole residential building (apartment building / housing company building), not just a single-family “house.” So koko talossa is naturally understood as “throughout the entire building.”

Finnish has no articles. Whether it’s “a” or “the” is inferred from context. In an existential sentence like Ilmoitustaululla on uusi ilmoitus, it’s typically understood as a new notice (introducing something new).

In Finnish existential sentences, the object-like noun can be:

• nominative (like uusi ilmoitus) when it’s seen as a complete, countable item and the sentence is affirmative
• partitive in negatives (ei ole ilmoitusta) or when meaning is indefinite mass/partial/ongoing

Here it’s one specific countable item: a new notice, so uusi ilmoitus stays nominative.

Huomenna is an adverb meaning tomorrow. It often appears near the beginning of the clause to set the time:

• Huomenna on vesikatko... (Tomorrow there is a water outage...)

But it can move for emphasis, e.g. Vesikatko on huomenna (The outage is tomorrow), which sounds more contrastive (“not today, but tomorrow”).