Sisäkumi puhkesi, koska rengas kului loppuun.

In Finnish, sisäkumi is the inner tube, and rengas is the tire (the outer rubber part).

So the sentence is talking about two different parts:

• sisäkumi = the inner tube
• rengas = the outer tire

That is why both words appear: the sentence says that the inner tube punctured/burst because the tire wore out.

The basic dictionary form is puhjeta.

Here, puhkesi is the third-person singular past tense:

• puhjeta = to burst, to get punctured
• puhkesi = burst / got punctured

So Sisäkumi puhkesi means The inner tube got punctured or The inner tube burst.

This is just how this verb behaves in Finnish. Some verbs have a past tense in -si.

For puhjeta, the past tense stem becomes:

• puhjeta → puhkesi

This is something learners usually need to memorize as part of the verb pattern. It is not fully predictable from English.

A useful takeaway is:

• puhjeta = infinitive
• puhkesi = past tense, he/she/it burst

The basic form is kulua.

In this sentence:

• kulua = to wear down, to wear out
• kului = wore down / wore out

So:

• rengas kului = the tire wore out

This is again third-person singular past tense.

Loppuun adds the idea of completely, all the way, or until finished.

So:

• kului = wore down
• kului loppuun = wore out completely

This is a very common Finnish pattern: a verb plus a word that gives a sense of completion.

Examples of the idea:

• syödä loppuun = eat up completely
• juoda loppuun = drink up
• kulua loppuun = get worn out completely

So rengas kului loppuun is stronger and more complete than just rengas kului.

Yes. Loppuun is the illative form of loppu (end).

Literally, it has the sense of into the end, but in many expressions it works more naturally as completely, to the end, or out.

In this sentence, you do not need to translate it literally. The important thing is that:

• kulua loppuun = to wear out

So although the form is grammatical case-wise interesting, the whole phrase is best learned as a unit.

Because koska introduces a subordinate clause, similar to because in English.

The sentence has two parts:

1. Sisäkumi puhkesi
2. koska rengas kului loppuun

In standard Finnish writing, a comma is normally used before this kind of subordinate clause.

So the comma here is completely normal.

Not always. Koska can mean:

• because
• when

The meaning depends on context.

In this sentence, it clearly means because:

• Sisäkumi puhkesi, koska rengas kului loppuun.
• The inner tube punctured because the tire wore out.

A time meaning (when) would not make much sense here.

Because they are the subjects of their clauses.

• Sisäkumi puhkesi = the inner tube burst
• rengas kului loppuun = the tire wore out

In both parts, the noun is doing what the verb describes, so Finnish uses the nominative singular:

• sisäkumi
• rengas

There is no article like the or a in Finnish, so the plain nominative form is enough.

Finnish does not have articles like a, an, or the.

So:

• sisäkumi can mean an inner tube, the inner tube, or just inner tube
• rengas can mean a tire, the tire, or just tire

The exact meaning comes from context. In this sentence, English naturally uses the:

• The inner tube punctured, because the tire wore out.

Finnish often puts the subject first and the verb right after it in neutral statements.

So the structure here is very straightforward:

• Sisäkumi puhkesi
• rengas kului loppuun

This is one of the most neutral and common word orders in Finnish.

Finnish word order is more flexible than English, but this sentence uses a very standard pattern.

Yes. Finnish word order is flexible, so other versions are possible, for example:

• Koska rengas kului loppuun, sisäkumi puhkesi.

This means the same thing: Because the tire wore out, the inner tube punctured.

The original version starts with the result first and the reason second. The alternative starts with the reason.

Both are grammatical; the difference is mainly in focus and style.

Because the sentence describes completed events in the past.

• puhkesi = burst / got punctured
• kului loppuun = wore out

If you used the present tense, it would describe something happening now or something habitual, which would not fit this kind of statement as naturally.

So the past tense is the normal choice here.

Here it is intransitive.

That means the subject undergoes the event:

• Sisäkumi puhkesi = The inner tube burst / got punctured

No direct object is mentioned.

This is different from a transitive verb like puhkaista:

• Hän puhkaisi sisäkumin = He/She punctured the inner tube

So:

• puhjeta = to burst / get punctured
• puhkaista = to puncture something

That is a useful distinction for learners.

Yes, that is a very good idea.

While you can analyze the parts:

• kulua = wear down
• loppuun = to the end / completely

the combination kulua loppuun is a very natural Finnish expression meaning:

• to wear out
• to become worn out completely

Learning it as a chunk will help you understand and use similar expressions more naturally.