Ég nota kústinn þegar gólfið í eldhúsinu er fullt af rusli.

Because kústinn is the direct object of nota (to use), and nota normally takes the accusative case.

The noun is kústur (broom), a masculine noun:

• kústur = a broom, nominative singular
• kúst = a broom, accusative singular
• kústurinn = the broom, nominative singular
• kústinn = the broom, accusative singular

So in Ég nota kústinn, the form has to be accusative definite: kústinn.

In Icelandic, the definite article is usually attached to the end of the noun instead of being a separate word.

So here:

• kústinn = the broom
• gólfið = the floor
• eldhúsinu = the kitchen (in the dative form)

This is one of the first big differences English speakers notice. Icelandic usually does not say a separate word like English the before the noun.

Gólfið is the subject of er fullt af rusli (is full of trash), so it is in the nominative case.

The base noun is gólf (floor), which is a neuter noun. In the definite singular, gólfið means the floor.

A useful thing to know: for many neuter nouns, the nominative and accusative forms are the same, so gólfið could look the same in both cases. Here, though, its job in the sentence is nominative because it is the subject.

Because í can take two different cases depending on meaning:

• dative for location: in
• accusative for motion into: into

Here the meaning is location: the floor in the kitchen, so Icelandic uses the dative:

• í eldhúsinu = in the kitchen

Compare:

• Ég er í eldhúsinu. = I am in the kitchen.
• Ég fer í eldhúsið. = I go into the kitchen.

So this is a very common pattern: static location = dative, movement toward/into = accusative.

Because the adjective has to agree with the noun it describes.

Here it describes gólfið (the floor), which is:

• neuter
• singular
• nominative

So the adjective fullur (full) must appear in the neuter singular form:

• masculine: fullur
• feminine: full
• neuter: fullt

That is why the sentence says gólfið er fullt af rusli.

Because the preposition af normally takes the dative case.

The noun is rusl (trash, rubbish, litter), and its dative singular form is rusli.

So:

• rusl = trash
• af rusli = of trash / with trash / full of trash

Also, in this sentence rusl is being used as a general mass noun, not as a specific piece or set of trash. That is why there is no definite article here.

So:

• fullt af rusli = full of trash / covered with a lot of trash

Here þegar means when or whenever.

Because the sentence describes a habitual situation, English might naturally understand it as whenever:

• Ég nota kústinn þegar... = I use the broom when/whenever...

So this is not only about one single occasion. It can describe a repeated situation: whenever the kitchen floor is full of trash, I use the broom.

It literally means the floor in the kitchen.

The noun comes first:

• gólfið = the floor

Then the prepositional phrase tells you which floor:

• í eldhúsinu = in the kitchen

So the structure is very similar to English the floor in the kitchen.

A native English speaker might also wonder whether Icelandic could say the kitchen floor as one word. Yes, very often it can:

• eldhúsgólfið = the kitchen floor

So both patterns are possible, though they are structured a little differently.

Að nota means to use, and it is a normal transitive verb. That means it takes a direct object, and that object is usually in the accusative case.

So:

• Ég nota kústinn. = I use the broom.
• Ég nota bílinn. = I use the car.

This is worth learning as a pattern: in Icelandic, you often need to know which case a verb governs, not just its basic meaning.

Yes, but the meaning would shift slightly.

• nota = use
• sópa = sweep

So:

• Ég nota kústinn... focuses on using the broom
• Ég sópa gólfið... focuses on sweeping the floor

If you want the more direct idea I sweep the kitchen floor when it is full of trash, Icelandic would often use sópa.

But the original sentence is perfectly good if the point is specifically I use the broom in that situation.

Then Icelandic uses normal verb-second word order in the main clause:

• Þegar gólfið í eldhúsinu er fullt af rusli, nota ég kústinn.

Notice that after the fronted þegar clause, the finite verb nota comes before ég.

This often surprises English speakers, because English would keep I use in that order, but Icelandic switches to nota ég after something else has been placed first.