Ég set sængina á rúmið.

Because setja is the infinitive form of the verb, meaning to put / to set. In the sentence, the verb has to be conjugated to match the subject ég (I).

So:

• setja = to put
• ég set = I put / I am putting

This is a normal present-tense pattern for this verb:

• ég set = I put
• þú setur = you put
• hann/hún/það setur = he/she/it puts

So ég set sængina á rúmið uses the correct 1st person singular present tense.

Sængina is the noun sæng with the definite article attached, and it is in the accusative singular.

Breakdown:

• sæng = duvet / quilt / comforter
• sængin = the duvet (nominative)
• sængina = the duvet (accusative)

Icelandic usually puts the at the end of the noun instead of using a separate word like English does.

So instead of the duvet, Icelandic says sængin or sængina, depending on the case.

In this sentence, it is accusative because it is the direct object of set.

It is in the accusative because it is the thing being put — in other words, the direct object of the verb.

In Ég set sængina á rúmið:

• Ég = subject
• set = verb
• sængina = direct object

Many Icelandic verbs take a direct object in the accusative, and setja is one of them.

So the sentence answers the question:

• What am I putting? → sængina

That is why sængina is accusative.

Because Icelandic uses different cases after many prepositions depending on whether there is motion or location.

With á:

• accusative = movement onto something
• dative = position on something

So:

• á rúmið = onto the bed
• á rúminu = on the bed

In your sentence, the action is dynamic: the duvet is being moved onto the bed. That is why Icelandic uses accusative:

• Ég set sængina á rúmið.

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

• Sængin er á rúminu. = The duvet is on the bed.

Rúmið is accusative singular definite of rúm (bed).

Breakdown:

• rúm = bed
• rúmið = the bed

In this sentence it is accusative because it comes after á in a phrase showing movement toward a destination:

• á rúmið = onto the bed

So even though rúmið is not the direct object of the main verb, it is still accusative because the preposition á requires accusative here.

You know from the case and from the meaning of the verb.

The verb setja involves placing something somewhere, so there is movement. With á, movement usually gives:

• á + accusative = onto

Location without movement gives:

• á + dative = on

So:

• Ég set sængina á rúmið. = I put the duvet onto the bed.
• Sængin er á rúminu. = The duvet is on the bed.

English often uses on for both ideas, but Icelandic shows the difference through case.

This is basically normal Icelandic word order for a simple statement:

• Ég = subject
• set = verb
• sængina = object
• á rúmið = prepositional phrase

So the pattern is:

Subject + Verb + Object + Place/Direction

That makes this sentence quite straightforward.

A very literal structure is:

• I put the duvet onto the bed

Icelandic can vary word order more than English in some contexts, but this sentence is a standard, neutral way to say it.

Because Icelandic usually expresses the as a suffix attached to the noun.

Examples from this sentence:

• sæng = duvet

• sængina = the duvet

• rúm = bed
• rúmið = the bed

This is one of the biggest differences from English. Instead of saying:

• the bed

Icelandic often says something like:

• bed-the

Of course, the exact ending changes depending on gender, number, and case, so learners need to memorize noun forms rather than expect one fixed word for the.

A good habit is to reduce the sentence to the key dictionary forms:

• ég = I
• setja = to put
• sæng = duvet / quilt / comforter
• á = on / onto
• rúm = bed

That helps you connect the sentence form to what you would actually find in a dictionary.

From the sentence itself:

• set comes from setja
• sængina comes from sæng
• rúmið comes from rúm

This is especially important in Icelandic because endings change a lot.

It is similar, but not exactly identical.

A learner-friendly approximation:

• Ég sounds roughly like yegg with a different vowel quality
• set is somewhat close to English set
• sængina has the æ vowel, which does not match English exactly
• rúmið has ú, which is more like a long oo sound, and ð is a soft sound somewhat like th in this

A few useful notes:

• g in ég is not pronounced like a hard English g
• æ in sængina is a distinct Icelandic vowel
• ð in rúmið is often very light

If your goal is grammar, pronunciation does not change the structure, but it is worth listening to native audio because Icelandic spelling is not always obvious to English speakers.

Yes, but the meaning would become a duvet and a bed, not the duvet and the bed.

For example:

• Ég set sæng á rúm.

That is grammatical in some contexts, but it sounds much less natural for an ordinary everyday situation unless you really mean something general or indefinite.

Usually, if you are talking about a specific duvet and a specific bed, the definite forms are more natural:

• sængina = the duvet
• rúmið = the bed

So the original sentence sounds like a normal real-life statement about specific objects.

Probably these three things:

1. Verb conjugation

   • setja → ég set

2. The definite article is attached to the noun

   • sængina, rúmið

3. Prepositions can change case depending on motion vs. location

   • á rúmið = onto the bed
   • á rúminu = on the bed

If you understand those three points, you are getting a lot of value from this one short sentence.