Ketjussa on ruostetta märän talven jälkeen.

Ketjussa is the inessive form of ketju (chain). The inessive ending -ssa/-ssä often means in, but in Finnish it is also used in some places where English would say on.

So:

• ketju = chain
• ketjussa = in the chain / on the chain

In this sentence, ketjussa on ruostetta means there is rust on/in the chain. English and Finnish do not always match exactly in these location expressions.

Finnish often uses to be (olla) in sentences that express existence.

So ketjussa on ruostetta literally looks like:

• in/on the chain
• is
• rust

This is a very common Finnish structure: [place] + on + [something] = there is [something] in/at/on [place].

For example:

• Pöydällä on kirja. = There is a book on the table.
• Ketjussa on ruostetta. = There is rust on the chain.

Ruostetta is the partitive singular of ruoste (rust).

Finnish often uses the partitive when talking about:

• an unspecified amount of something
• a substance or material
• something that is present, but not treated as a whole, countable item

Rust here is not being presented as one complete, separate object. It is just some rust or rust present on the chain, so the partitive is natural:

• ruoste = rust
• ruostetta = (some) rust

This is very common in existential sentences:

• Lasissa on vettä. = There is water in the glass.
• Maassa on lunta. = There is snow on the ground.
• Ketjussa on ruostetta. = There is rust on the chain.

Both are natural, but they focus on slightly different things.

• Ketjussa on ruostetta = There is rust on the chain.

  • This points to the presence of rust.
  • It can suggest some amount of rust.

• Ketju on ruosteessa = The chain is rusty / The chain is covered in rust.

  • This describes the state or condition of the chain.

So:

• if you want to mention that rust has appeared, ketjussa on ruostetta works well
• if you want to describe the chain as rusty, ketju on ruosteessa is often more direct

Because jälkeen (after) is a postposition, and it requires the word before it to be in the genitive.

Here the phrase is:

• märkä talvi = a wet winter
• märän talven = of a wet winter / a wet winter (genitive form used before jälkeen)
• märän talven jälkeen = after a wet winter

Both the adjective and the noun must agree:

• märkä → märän
• talvi → talven

This is normal in Finnish noun phrases.

Because jälkeen is a postposition, not a preposition.

English uses prepositions, which come before:

• after the winter

Finnish often uses postpositions, which come after:

• talven jälkeen = after the winter

Some Finnish location/time words work this way, so learners often need time to get used to the word order.

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

So Finnish simply says:

• ketjussa on ruostetta märän talven jälkeen

and context tells you whether English should use:

• rust
• some rust
• the chain
• a wet winter
• the wet winter

This is very normal in Finnish. You usually understand definiteness from the situation, not from an article.

This is a very common Finnish word order for an existential sentence.

Finnish often puts the place or setting first, and then says what exists there:

• Ketjussa on ruostetta.
• literally: On the chain there is rust.

This sounds natural because the sentence is presenting rust as something found in that location.

You could change the word order for emphasis, but the given order is standard and natural.

Yes, that is grammatically possible, but it changes the emphasis.

• Ketjussa on ruostetta... focuses first on the chain as the location
• Ruostetta on ketjussa... focuses first on rust

The original sentence is the more neutral, natural choice if you are simply stating what is on the chain.

Primarily it expresses time: after a wet winter.

But in context, it may also suggest a likely reason or background cause. A wet winter often leads to rust, so the phrase can imply both:

• when this happened: after a wet winter
• why it may have happened: because conditions were wet

Grammatically, though, it is a time expression.

Yes. That is one of the common functions of the partitive.

In this sentence, ruostetta naturally suggests:

• rust in general
• some rust
• an unspecified amount of rust

So even without a separate word for some, Finnish communicates that meaning through the case ending.

They are:

• ketjussa → ketju = chain
• on → olla = to be
• ruostetta → ruoste = rust
• märän → märkä = wet
• talven → talvi = winter
• jälkeen = after

This is a useful habit in Finnish: when you see an inflected form, try to identify the base form and then the ending.