Poljin vaurioitui, kun pyörä kaatui.

Vaurioitui is the 3rd person singular past tense of vaurioitua, which means to get damaged or to become damaged.

So:

• vaurioitua = to get damaged
• vaurioitui = got damaged / became damaged

It is not a passive form. The subject is poljin, so the sentence says that the pedal was the thing that became damaged.

Poljin is the subject of the main clause, so it appears in the nominative singular, which is the basic dictionary form.

In Poljin vaurioitui:

• poljin = the pedal
• vaurioitui = got damaged

This works much like English: The pedal got damaged.

Because the pedal is the thing that underwent the change, Finnish uses it as the subject, not as an object.

Not really. Finnish does not mean that the pedal deliberately damaged itself.

Vaurioitua is an intransitive change-of-state verb. It means that something ends up damaged.

Compare:

• Poljin vaurioitui = The pedal got damaged
• Joku vaurioitti poljinta = Someone damaged the pedal

So:

• vaurioitua = become damaged
• vaurioittaa = damage something

This is a very common Finnish pattern: one verb means something changes state, and a related verb means someone causes that change.

Kaatui is the 3rd person singular past tense of kaatua, which means to fall, to topple, or to fall over.

So:

• kaatua = to fall over
• kaatui = fell over

In this sentence, pyörä kaatui means that the bike toppled over.

Because pyörä is the subject of the subordinate clause kun pyörä kaatui.

That clause works like this:

• kun = when
• pyörä = the bike
• kaatui = fell over

So again, the thing doing the action of falling is in the nominative.

On its own, pyörä can mean either wheel or bicycle/bike, depending on context.

In this sentence, the natural meaning is bike, because kun pyörä kaatui strongly suggests when the bike fell over.

If it meant wheel, the context would usually make that clearer.

Kun is a very common word meaning when. It introduces a time clause.

So:

• Poljin vaurioitui = The pedal got damaged
• kun pyörä kaatui = when the bike fell over

Together, the second clause tells when the damage happened.

In past-time narration, kun often means when in the sense of at the time that or when this happened.

Yes. The exact English wording depends on style and context.

Kun mainly marks the time relationship. In English, that may come out as:

• when
• as
• sometimes something like when the bike fell over

But the Finnish itself is straightforwardly temporal here. It is not especially dramatic or formal.

Because the sentence describes a past event, and both parts happened in that past situation:

• Poljin vaurioitui = the pedal got damaged
• kun pyörä kaatui = when the bike fell over

Finnish commonly uses the simple past in both clauses like this. English often does the same:

• The pedal got damaged when the bike fell over

Nothing more complicated is required here.

Yes, Finnish word order is fairly flexible, although the version given is the most neutral.

For example, you could also say:

• Kun pyörä kaatui, poljin vaurioitui.

This means the same thing, but it starts with the time clause, which slightly foregrounds the event of the bike falling over.

So:

• Poljin vaurioitui, kun pyörä kaatui. = neutral, main event first
• Kun pyörä kaatui, poljin vaurioitui. = time/background first

Good question. Both can be possible in some contexts, but they are not exactly the same.

• vaurioitui = got damaged
• rikkoutui = broke

Vaurioitui is a bit broader and does not necessarily mean the pedal stopped functioning completely. It may have been scratched, bent, dented, or otherwise damaged.

Rikkoutui usually suggests that something actually broke or became unusable.

So vaurioitui is a safer, more general choice if the damage is not total.

Yes. Finnish can often replace a kun clause with another structure, especially in written language. For example, you might see something like a form built from kaatua, but that is more advanced and less neutral for a beginner.

The sentence you have is a very natural and clear everyday way to say it, so it is a good pattern to learn first:

• main clause
• kun
  • past clause

That structure is extremely useful in Finnish.