Vesivahinko aiheutui vuotavasta putkesta.

It breaks down like this:

• vesivahinko = water damage
• aiheutui = was caused / resulted
• vuotavasta putkesta = from a leaking pipe

So the structure is roughly:

[thing that happened] + [was caused] + [source/cause in the elative case]

A very literal translation would be:

Water damage resulted from a leaking pipe.

The dictionary form is aiheutua.

This verb means to be caused, to result from, or to arise from something. It is an intransitive verb, which means it does not take a direct object in this use.

In the sentence:

• aiheutui = past tense
• third person singular
• from aiheutua

So:

• aiheutuu = is caused / results
• aiheutui = was caused / resulted

Because vesivahinko is the subject, and the verb agrees with it in the normal third person singular form.

• vesivahinko = singular subject
• therefore aiheutui = singular past tense

Finnish does not need a passive here. Even though English often says was caused, Finnish commonly uses the intransitive verb aiheutua instead.

So Finnish is not literally using the same kind of passive structure as English. It is more like:

• The water damage resulted from a leaking pipe

rather than

• The water damage was caused by a leaking pipe

Even though both are good English translations.

Because putkesta is in the elative case, which usually means out of, from inside, or more generally from.

• putki = pipe
• putkesta = from the pipe

With the verb aiheutua, the cause or source is often marked with the elative:

• tulipalo aiheutui kynttilästä = the fire was caused by a candle
• ongelma aiheutui virheestä = the problem resulted from an error

So in your sentence:

• vuotavasta putkesta = from a leaking pipe

Because vuotava describes putki, and in Finnish modifiers must agree with the noun they modify.

Here:

• putkesta is in the elative case
• so the describing word must also be in the elative case

That gives:

• vuotava putki = a leaking pipe
• vuotavasta putkesta = from a leaking pipe

So both words carry the same case ending:

• vuotava → vuotavasta
• putki → putkesta

This agreement is very normal in Finnish.

Vuotava is a present active participle from the verb vuotaa (to leak).

It works very much like an adjective:

• vuotava putki = a leaking pipe
• vuotava katto = a leaking roof

So although it comes from a verb, in this sentence it behaves like a descriptive word.

A useful pattern is:

• -va / -vä participle = doing X / that does X

Examples:

• nukkuva lapsi = a sleeping child
• haukkuva koira = a barking dog
• vuotava putki = a leaking pipe

Yes. Finnish very often forms compound nouns where English uses two separate words.

• vesi = water
• vahinko = damage, accident, harm
• vesivahinko = water damage

This is completely normal Finnish word formation. Other common examples:

• kahvikuppi = coffee cup
• sadesää = rainy weather
• talonmies = caretaker / janitor depending on context

So vesivahinko is not unusual at all; it is a standard compound noun.

Because Finnish does not have articles like English a/an and the.

So:

• putki can mean a pipe or the pipe
• vesivahinko can mean water damage, the water damage, or sometimes a water-damage incident, depending on context

The exact meaning is understood from context, not from articles.

That is why vuotavasta putkesta can be translated as:

• from a leaking pipe
• from the leaking pipe

depending on the situation.

No, Finnish word order is fairly flexible, although this sentence has a very natural neutral order.

Neutral version:

• Vesivahinko aiheutui vuotavasta putkesta.

You could also say:

• Vuotavasta putkesta aiheutui vesivahinko.

Both are grammatical. The difference is mainly in emphasis or information flow.

• Starting with vesivahinko puts the result first
• Starting with vuotavasta putkesta puts the cause first

Finnish often uses word order to highlight what is already known and what is new information.

Yes, often you could also use johtui from johtua.

For example:

• Vesivahinko johtui vuotavasta putkesta.

This also means The water damage was due to / resulted from a leaking pipe.

The difference is small in many contexts:

• aiheutua focuses a bit more on being caused
• johtua often feels a bit more like be due to / stem from

In many practical situations, both are possible.

It can be. That is a different but closely related sentence.

• Vuotava putki aiheutti vesivahingon.
• A leaking pipe caused the water damage.

This version uses the transitive verb aiheuttaa (to cause), not aiheutua (to be caused / result from).

Compare:

• vesivahinko aiheutui vuotavasta putkesta
  = the water damage was caused by / resulted from a leaking pipe

• vuotava putki aiheutti vesivahingon
  = the leaking pipe caused the water damage

So Finnish can express the same situation from either direction, just as English can.

It is in the nominative singular, which is the basic dictionary-form case for nouns.

Here it functions as the subject of the sentence:

• vesivahinko = the thing that happened
• aiheutui = what happened to it
• vuotavasta putkesta = the source/cause

So this is a very typical Finnish sentence pattern:

• subject in nominative
• verb
• source/cause in elative

The present tense would be:

• Vesivahinko aiheutuu vuotavasta putkesta.

That means:

• The water damage is caused by a leaking pipe
• or Water damage results from a leaking pipe

So the only change is:

• aiheutui = past
• aiheutuu = present

Everything else stays the same.