Voimme kunnostaa pyörän, mutta ketjussa on ruostetta ja se pitää vaihtaa.

Voimme is the 1st person plural present tense of voida (to be able to / can).

• minä voin = I can
• me voimme = we can

So Voimme kunnostaa... means We can fix up / restore...

Finnish often puts the finite verb early in the clause, so this is a very natural opening.

Kunnostaa means something like to restore, renovate, put into good condition, fix up.

It is close to korjata (to repair / fix), but there is a slight nuance:

• korjata = repair something that is broken
• kunnostaa = make something serviceable or good again, often more generally

In this sentence, kunnostaa pyörän suggests getting the bike back into proper condition, not just one tiny repair.

Because pyörän is the total object form here.

With an affirmative sentence like this, when the object is understood as a complete whole, Finnish often uses:

• singular genitive-looking form: pyörän
• rather than the basic form pyörä

So:

• Voimme kunnostaa pyörän = We can restore/fix the bike as a whole

If the meaning were more partial, ongoing, or incomplete, Finnish might use the partitive instead.

Also, in context pyörä means bicycle, although the word can also mean wheel in other contexts.

Finnish has no articles, so it does not have direct equivalents of a/an and the.

Whether something is understood as a bike, the bike, a chain, or the chain comes from:

• context
• word order
• case endings
• what is already known in the conversation

So pyörän can mean the bike here because the context makes it clear.

Ketjussa is the word ketju (chain) in the inessive case, which usually means in.

• ketju = chain
• ketjussa = in the chain

Finnish often expresses this idea as there is rust in the chain, even though English would usually say the chain has rust or there is rust on the chain.

The ending -ssa / -ssä is one of the common Finnish location endings.

Both are possible ideas, but they mean slightly different things.

• ketjussa on ruostetta = there is rust in/on the chain
• ketju on ruosteinen = the chain is rusty

The version in your sentence focuses on the presence of rust as a substance/problem. That works well because the next clause explains the consequence: it needs to be replaced.

Ruostetta is the partitive singular of ruoste (rust).

It is used here because rust is being treated as an uncountable substance or an indefinite amount:

• on ruostetta = there is some rust

This is very common in Finnish existential sentences:

• Pöydällä on vettä. = There is some water on the table.
• Ketjussa on ruostetta. = There is some rust in/on the chain.

If you used ruoste instead, it would sound more like referring to rust as a definite, whole thing, which is not the natural choice here.

Here se refers to the chain.

Even though the previous clause says ketjussa (in the chain) rather than plain ketju, the listener understands that the problem is with the chain itself. So:

• ketjussa on ruostetta
• ja se pitää vaihtaa

= there is rust in the chain, and it needs to be replaced

In Finnish, se is very commonly used for things and also very often for people in everyday speech.

No. Here pitää does not mean to hold.

In this structure, pitää + infinitive expresses necessity:

• pitää vaihtaa = must be replaced / needs to be replaced or one must replace

So in this sentence:

• se pitää vaihtaa

means it has to be replaced.

This is a very common Finnish way to express must / have to.

Because after pitää in the meaning of must / have to, the next verb appears in the basic infinitive form.

So:

• pitää vaihtaa = must replace / must be replaced
• pitää korjata = must repair
• pitää ostaa = must buy

If you said vaihdetaan, that would be a different structure:

• Se vaihdetaan. = It is replaced / It will be replaced / We’ll replace it (depending on context)

But after pitää, Finnish normally wants the infinitive:

• Se pitää vaihtaa.

That is normal in Finnish.

The structure pitää vaihtaa often leaves the doer unspecified when the point is simply that something is necessary.

So se pitää vaihtaa can mean:

• it must be replaced
• it needs to be replaced
• someone has to replace it
• we have to replace it

The exact doer is understood from context, and in many situations it does not need to be stated.

The word order here is quite natural and neutral.

• Voimme kunnostaa pyörän = normal main clause
• mutta ketjussa on ruostetta = existential-style clause, with the location first
• ja se pitää vaihtaa = normal continuation

Finnish word order is more flexible than English, but this version sounds straightforward and natural.

The placement of ketjussa before on ruostetta helps present the problem as in the chain, there is rust. That is a very common Finnish pattern.

They connect different relationships between the ideas.

• mutta = but
• ja = and

So the sentence structure is:

1. We can fix up the bike,
2. but there is a problem:
   • there is rust in the chain
3. and as a result,
   • it needs to be replaced

Using both conjunctions makes the logic of the sentence clear and natural.