Pesutuvan vieressä on kuivaushuone, jossa pyykit kuivuvat nopeasti.

Because vieressä works like a postposition meaning next to / beside, and postpositions in Finnish usually require the noun before them to be in the genitive.

So:

• pesutupa = laundry room
• pesutuvan = of the laundry room / laundry room’s

Together:

• pesutuvan vieressä = next to the laundry room

This is similar to how English says next to the laundry room, where next to forms one unit. In Finnish, vieressä comes after the noun, so the noun takes the genitive form.

Both are possible, but they have slightly different feel.

• Pesutuvan vieressä on kuivaushuone presents the place first and then introduces what is there. This is a very common Finnish existential pattern:

  • [place] + on + [something]
  • literally: Next to the laundry room is a drying room

• Kuivaushuone on pesutuvan vieressä is also grammatical, but it sounds more like you are talking about a drying room that is already known and then saying where it is:

  • The drying room is next to the laundry room

So the original sentence is natural when introducing the drying room as new information.

Jossa means in which or more naturally where.

It comes from the relative pronoun joka:

• joka = who / which / that
• jossa = in which / where

Here it refers back to kuivaushuone:

• kuivaushuone, jossa...
• a drying room, where...
• more literally: a drying room, in which...

So:

• Pesutuvan vieressä on kuivaushuone, jossa pyykit kuivuvat nopeasti.
• Next to the laundry room there is a drying room where the laundry dries quickly.

Because pyykit means the laundry / the clothes being washed, and in Finnish this is commonly expressed as a plural word.

The basic form is:

• pyykki = a piece of laundry / laundry item

Plural:

• pyykit = the laundry, the clothes, the washing

So even if English often uses singular-looking laundry, Finnish often uses a plural form here.

Here pyykit is the subject of the clause pyykit kuivuvat nopeasti.

That clause is a normal statement:

• pyykit = the laundry / the clothes
• kuivuvat = dry / are drying
• nopeasti = quickly

In a normal affirmative clause like this, the subject is usually in the nominative, so pyykit is correct.

The partitive plural pyykkejä would suggest a different structure or meaning, often something more indefinite or partial. In this sentence, we are simply talking about the laundry as the subject that dries quickly, so pyykit is the natural choice.

Because Finnish has two related verbs here:

• kuivua = to dry, to become dry
  
  • intransitive: no object
• kuivata = to dry something
  
  • transitive: takes an object

In this sentence, the laundry is becoming dry, so Finnish uses kuivua:

• pyykit kuivuvat = the clothes dry / are drying

If someone is drying the clothes, then you would use kuivata:

• Minä kuivaan pyykit. = I dry the clothes.
• Aurinko kuivattaa pyykit. = The sun dries the clothes.

So kuivuvat is correct because the clothes themselves are the subject that become dry.

Because the subject pyykit is plural.

Finnish verbs agree with the subject:

• pyykki kuivuu = the item of laundry dries
• pyykit kuivuvat = the clothes dry

So:

• singular subject → kuivuu
• plural subject → kuivuvat

The ending -vat/-vät is the normal 3rd person plural ending in Finnish.

Nopeasti is an adverb meaning quickly.

It is formed from the adjective:

• nopea = fast, quick

Adverb form:

• nopeasti = quickly

A very common way to make adverbs in Finnish is to add -sti to an adjective stem.

So here:

• pyykit kuivuvat nopeasti = the clothes dry quickly

It is a compound noun, which is very common in Finnish.

It consists of:

• kuivaus = drying
• huone = room

So:

• kuivaushuone = drying room

Finnish often combines words directly into one longer noun. The same happens with:

• pesutupa
  • pesu = washing
  • tupa = room/building/utility room
  • pesutupa = laundry room

Learning to spot compounds is very helpful in Finnish, because long words are often just smaller familiar words joined together.

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

So:

• kuivaushuone can mean a drying room or the drying room
• pesutupa can mean a laundry room or the laundry room

The exact meaning depends on context, word order, and what the listener already knows.

In this sentence, English usually translates it as:

• Next to the laundry room there is a drying room...

That sounds natural in English, even though Finnish does not separately mark the and a.