Likaiset vaatteet ovat pyykkikorissa, mutta puhtaat pyykit ovat vielä kuivausrummussa.

There isn’t one. Finnish has no articles, so it does not use separate words for the or a/an.

In this sentence, definiteness is understood from context:

• Likaiset vaatteet = the dirty clothes
• puhtaat pyykit = the clean laundry

So Finnish often leaves that kind of information unstated unless something else in the sentence makes it clear.

Because the sentence is talking about more than one item.

• vaate = a piece of clothing
• vaatteet = clothes
• pyykki can mean laundry, but pyykit often refers to laundry items or a load of laundry

The adjectives are plural too:

• likainen → likaiset
• puhdas → puhtaat

And the verb is plural as well:

• on = is
• ovat = are

So the whole sentence agrees in number.

Because those words are related, but not exactly the same.

• vaatteet means clothes
• pyykki / pyykit means laundry

So:

• Likaiset vaatteet focuses on the items as clothes
• puhtaat pyykit focuses on them as laundry that has been washed

That is very natural in Finnish. English also does something similar: we might say the dirty clothes but the clean laundry.

The ending -ssa / -ssä is the inessive case, which usually means in.

So:

• pyykkikori = laundry basket
• pyykkikorissa = in the laundry basket

Finnish often uses case endings where English uses prepositions. Instead of a separate word for in, Finnish adds an ending to the noun.

For the same reason: it means in the dryer.

• kuivausrumpu = tumble dryer / drying drum
• kuivausrummussa = in the tumble dryer

Again, -ssa gives the meaning in.

So both location phrases work the same way:

• pyykkikorissa = in the laundry basket
• kuivausrummussa = in the tumble dryer

Because of consonant gradation, a very common sound change in Finnish.

The basic word is:

• rumpu = drum

But in some forms, mp changes to mm:

• rumpu
• rummussa = in the drum

That same change happens inside the compound word:

• kuivausrumpu
• kuivausrummussa

So this is not random spelling; it follows a regular Finnish pattern.

Here vielä means still.

So:

• ovat vielä kuivausrummussa = are still in the dryer

It shows that the clean laundry has not been taken out yet.

A useful thing to know:

• in positive statements, vielä often means still
• in negative statements or questions, it often corresponds to yet

For example:

• Ne ovat vielä siellä. = They are still there.
• Ne eivät ole vielä täällä. = They are not here yet.

Because that is the normal position in Finnish.

So:

• likaiset vaatteet = dirty clothes
• puhtaat pyykit = clean laundry

Like in English, the adjective usually comes before the noun in ordinary descriptions.

Finnish word order is somewhat flexible, but putting the adjective before the noun is the standard, neutral pattern.

Because Finnish adjectives usually agree with the nouns they describe.

That means they match the noun in number and often also in case.

Here both noun phrases are plural subjects, so both adjectives are also plural:

• likainen vaate → likaiset vaatteet
• puhdas pyykki → puhtaat pyykit

This agreement is very important in Finnish grammar.

Because the sentence has two full clauses joined by mutta = but:

• Likaiset vaatteet ovat pyykkikorissa
• mutta puhtaat pyykit ovat vielä kuivausrummussa

Each clause has its own subject and verb, so repeating ovat is normal and natural.

English does the same thing:

• The dirty clothes are in the laundry basket, but the clean laundry is still in the dryer.

Yes. Finnish often uses pyykki and pyykit in ways that do not match English exactly.

• pyykki can mean laundry in a general sense
• pyykit can mean laundry items, the washing, or a load of laundry

So puhtaat pyykit is a very natural Finnish expression, even though English would usually say the clean laundry, not the clean laundries.

This is a vocabulary difference, not a mistake.

Yes. Finnish uses compound words very often.

• pyykki
  • kori → pyykkikori = laundry basket
• kuivaus
  • rumpu → kuivausrumpu = tumble dryer

Then the case ending is added to the whole compound:

• pyykkikorissa
• kuivausrummussa

This is one reason Finnish words can look long: several meaningful parts are joined into one word.

Yes, Finnish word order is more flexible than English word order.

The given sentence has a very neutral structure:

• Likaiset vaatteet ovat pyykkikorissa, mutta puhtaat pyykit ovat vielä kuivausrummussa.

But other orders are possible for emphasis, for example:

• Pyykkikorissa ovat likaiset vaatteet...

That would put more focus on the location.

So Finnish word order is flexible, but not random. Different orders create different emphasis.