Keittiö on täynnä roskaa.

It breaks down like this:

• keittiö = kitchen
• on = is
• täynnä roskaa = full of trash

So the sentence pattern is:

[subject] + on + täynnä + [partitive noun]

A very literal gloss would be:

Kitchen is full of trash.

Because täynnä normally takes the partitive case.

• base form: roska = trash, rubbish, a piece of trash
• partitive singular: roskaa

So after täynnä, Finnish says:

• täynnä vettä = full of water
• täynnä ihmisiä = full of people
• täynnä roskaa = full of trash

Using roska here would be ungrammatical in standard Finnish.

It is partitive singular.

That may seem strange to an English speaker, because trash can feel like “many things,” but in Finnish roskaa here treats trash as a mass or substance, not as countable individual items.

So:

• roskaa = trash in general, as a mass
• roskia = pieces/items of trash, partitive plural

Both can be possible in some contexts, but roskaa is the most natural general choice in this sentence.

They are related, but they are not used in exactly the same way.

• täysi = full, a regular adjective
• täynnä = full of, used especially in the pattern olla täynnä + partitive

For example:

• Lasi on täysi. = The glass is full.
• Lasi on täynnä vettä. = The glass is full of water.

So in your sentence, täynnä is the natural form because the sentence says what the kitchen is full of.

Because keittiö is the subject of the sentence: the kitchen is full of trash.

If you said keittiössä, that would mean in the kitchen, and the sentence would become a different kind of idea.

Compare:

• Keittiö on täynnä roskaa. = The kitchen is full of trash.
• Keittiössä on roskaa. = There is trash in the kitchen.

The first means the whole kitchen is in a messy state.
The second only says that some trash exists there.

Because Finnish has no articles.

English distinguishes between:

• a kitchen
• the kitchen

Finnish usually just says keittiö, and the exact meaning depends on context.

So Keittiö on täynnä roskaa can mean:

• The kitchen is full of trash
• A kitchen is full of trash

In real use, context normally makes it clear which one is meant.

Yes, Finnish word order is fairly flexible, but Keittiö on täynnä roskaa is the normal, neutral order.

That order is best for learners:

• Keittiö = subject
• on = verb
• täynnä roskaa = predicate/complement

Other orders are possible for emphasis, but they are more marked and less basic. If you are learning the pattern, stick with:

Keittiö on täynnä roskaa.

Yes, sometimes, but the nuance changes a little.

• täynnä roskaa = full of trash, garbage in general
• täynnä roskia = full of pieces of trash, bits of garbage

In many situations, roskaa sounds more natural because trash is often treated as an uncountable mass, like water or snow in similar structures.

Yes, very common. You can use:

[thing/place] + on + täynnä + partitive

Examples:

• Huone on täynnä ihmisiä. = The room is full of people.
• Lattia on täynnä vettä. = The floor is full of water.
• Piha on täynnä lunta. = The yard is full of snow.
• Pöytä on täynnä kirjoja. = The table is full of books.

So this sentence is a very useful model to remember.

A few helpful points:

• Finnish stress is usually on the first syllable.
• Double letters are pronounced longer:
  • tt in keittiö
  • nn in täynnä
  • aa in roskaa
• ä and ö are real vowel sounds in Finnish, not just spelling variants.

Roughly:

• keittiö → KEIT-ti-ö
• täynnä → TÄYN-nä
• roskaa → ROS-kaa

The most important thing for learners is to notice vowel length and consonant length, because they matter in Finnish.