Vesivahinko tapahtui viime yönä.

Because Finnish usually writes compound nouns as one word.

• vesi = water
• vahinko = damage, accident, mishap

So vesivahinko is literally a water-damage type of event. English often uses two words, but Finnish normally joins them into one compound.

No. Finnish does not have articles like a, an, or the.

So vesivahinko can mean:

• a water leak / a case of water damage
• the water leak / the water damage

The exact meaning depends on context.

The subject is vesivahinko.

It is in the nominative singular, and the verb tapahtui is also singular, so they match.

A very literal structure is:

• Vesivahinko = water damage / a water-damage incident
• tapahtui = happened
• viime yönä = last night

So the sentence is built very directly: Water damage happened last night.

Tapahtui is the past tense, third person singular form of tapahtua, which means to happen or to occur.

So:

• tapahtua = to happen
• tapahtui = happened

The subject vesivahinko is singular, so the verb is singular too.

Because viime yönä refers to a specific finished time in the past, and Finnish normally uses the simple past in that situation.

So:

• Vesivahinko tapahtui viime yönä = a normal, natural way to say it
• Vesivahinko on tapahtunut viime yönä = much less natural in ordinary Finnish

The perfect on tapahtunut is more natural when the exact finished time is not the main point, or when the present result is being emphasized.

This is a common Finnish time expression.

The form yönä is the essive case, which is often used in expressions of time meaning during, on, or in a certain time period.

So:

• yönä = during the night
• viime yönä = during the last night = last night

This is very common in Finnish:

• tänä aamuna = this morning
• viime vuonna = last year
• ensi viikolla = next week

Finnish often uses special case forms instead of separate prepositions.

Because yö changes its form when case endings are added.

This noun has a stem pattern you have to learn:

• yö = night
• yön = of the night
• yötä = night, at night, for a night
• yönä = during the night

So yönä is not random; it is just the essive form of yö.

A good way to learn it is as a small pattern rather than trying to force it into English-style logic.

Because viime usually stays unchanged in these common time expressions.

So Finnish says:

• viime yönä = last night
• viime viikolla = last week
• viime vuonna = last year

Do not confuse viime with viimeinen.

• viime = last, previous
• viimeinen = last, final

This creates an important difference:

• viime yönä = last night
• viimeisenä yönä = on the final night

So the sentence uses viime yönä because it means last night, not on the final night.

Yes. Finnish word order is flexible.

The given sentence is neutral and natural:

• Vesivahinko tapahtui viime yönä.

But you could also say:

• Viime yönä tapahtui vesivahinko.

That version puts more focus on last night.

Both are grammatical. Finnish often changes word order to change emphasis rather than basic meaning.

A simple learner-friendly breakdown is:

• ve-si-va-hin-ko
• ta-pah-tui
• vii-me
• yö-nä

A few helpful points:

• Stress is on the first syllable of each word.
• yö is one syllable.
• y is a front rounded vowel, similar to German ü or French u.
• ö is similar to German ö or French eu.
• ui in tapahtui is pronounced as a vowel combination, roughly oo-ee said smoothly.

So a rough pronunciation guide is:

VE-si-VA-hin-ko TAH-pah-tui VII-me YÖ-nä