Settu pottaplöntuna upp á gluggakistuna.

Settu is the singular informal imperative of að setja (to put, place, set).

So the speaker is giving a command to one person: Settu ... = Put ...

If you were speaking to more than one person, or using the polite/plural form, you would usually say: Setjið pottaplöntuna upp á gluggakistuna.

Because pottaplöntuna is the direct object of the verb setja, and here it is:

• singular
• definite (the potted plant)
• in the accusative case

The base word is pottaplanta. Its accusative singular indefinite form is pottaplöntu, and with the definite article added, it becomes pottaplöntuna.

The vowel change a → ö happens because this noun follows the pattern of planta, which changes in these forms:

• planta
• plöntu
• plöntuna

Yes. Both are compound nouns, which are very common in Icelandic.

• pottaplanta = pot
  • plant
• gluggakista = window
  • sill

A useful rule is that in Icelandic compounds, the last part is the main noun, and that is the part that usually determines the gender and declension.

So:

• pottaplanta behaves grammatically like planta
• gluggakista behaves grammatically like kista

That is why their endings change the way they do.

Here upp á means something like up onto or up on.

In this sentence:

• upp adds a sense of upward movement
• á means onto/on

So the full phrase suggests placing the plant up onto the windowsill.

In English, we would often just say put it on the windowsill, even though Icelandic uses this extra directional feeling more explicitly.

Because á can take different cases depending on the meaning:

• accusative = motion to a place
• dative = location at a place

Here the plant is being moved onto the windowsill, so Icelandic uses the accusative:

• upp á gluggakistuna = onto the windowsill

If you were describing where the plant already is, you would use the dative:

• Pottaplantan er á gluggakistunni.
• The potted plant is on the windowsill.

So this is a very common Icelandic pattern:

• movement → accusative
• position/location → dative

Because Icelandic usually puts the definite article (the) at the end of the noun, not as a separate word.

So:

• pottaplöntuna = the potted plant
• gluggakistuna = the windowsill

In this sentence, both nouns are:

• singular
• definite
• accusative
• feminine

That is why they both have similar endings.

You can compare:

• pottaplöntu = a potted plant

• pottaplöntuna = the potted plant

• gluggakistu = a windowsill
• gluggakistuna = the windowsill

This is a very natural Icelandic word order for a command:

• Settu — verb first
• pottaplöntuna — the thing being moved
• upp á gluggakistuna — where it should go

So the structure is basically:

Verb + direct object + destination phrase

That is normal and neutral here. Icelandic word order can sometimes be flexible, but this sentence sounds straightforward and idiomatic.

Often, yes. You could say:

Settu pottaplöntuna á gluggakistuna.

That would still be understandable and natural in many situations.

But upp adds a more vivid sense of putting it up onto the sill, which fits nicely because a windowsill is typically thought of as a slightly raised place.

So:

• á gluggakistuna = fine
• upp á gluggakistuna = a bit more specific/idiomatic in this context

You mostly know this from the sentence structure and the roles of the words.

• setja usually takes a direct object: the thing being placed
• upp á + place gives the destination

So:

• pottaplöntuna = the object being moved
• upp á gluggakistuna = where it is being moved to

Even though both nouns are in the accusative, they are doing different jobs:

• one is the object of the verb
• the other is the object of the preposition

This is completely normal in Icelandic.