Barnið sefur betur þegar bangsinn og dúkkan eru í rúminu.

-ið is the definite article attached to the noun, so barnið means the child.

In Icelandic, the word for child is barn.

• barn = a child
• barnið = the child

This is very common in Icelandic: instead of using a separate word like English the, Icelandic often adds the article to the end of the noun.

Sefur is the correct 3rd person singular present tense form of the verb sofa (to sleep).

So:

• að sofa = to sleep
• ég sef = I sleep / am sleeping
• þú sefur = you sleep
• barnið sefur = the child sleeps

This verb changes its stem in the present tense, so you cannot just take sofa and remove the -a.

Betur is the comparative adverb meaning better.

It comes from vel (well):

• vel = well
• betur = better
• best = best

Since it describes how the child sleeps, Icelandic uses an adverb, not an adjective.

Compare:

• Barnið sefur vel. = The child sleeps well.
• Barnið sefur betur. = The child sleeps better.

Here þegar means when and introduces a subordinate clause.

So:

• Barnið sefur betur þegar ... = The child sleeps better when ...

It connects the main statement with the condition or situation in which it is true.

In many sentences, þegar works much like English when.

Because both nouns are definite:

• bangsi = a teddy bear

• bangsinn = the teddy bear

• dúkka = a doll
• dúkkan = the doll

Again, Icelandic usually puts the definite article on the end of the noun.

The exact ending changes depending on the noun’s gender and form:

• masculine nouns often get endings like -inn
• feminine nouns often get endings like -in or -an
• neuter nouns often get -ið

So the different endings are normal.

Because the subject is plural: bangsinn og dúkkan = the teddy bear and the doll.

Since there are two things, Icelandic uses the plural form of to be:

• er = is
• eru = are

So:

• bangsinn er í rúminu = the teddy bear is in the bed
• bangsinn og dúkkan eru í rúminu = the teddy bear and the doll are in the bed

Because í can take different cases depending on whether there is location or movement.

Here the meaning is in the bed, meaning a position or location, so Icelandic uses the dative:

• í rúminu = in the bed

If the meaning were into the bed or getting into the bed, Icelandic would use the accusative:

• í rúmið = into the bed

This is a very important Icelandic pattern:

• location → often dative
• motion toward/into → often accusative

Rúminu is dative singular definite of rúm (bed).

The forms are:

• rúm = a bed
• rúmið = the bed
• í rúmi / í rúminu = in a bed / in the bed

In this sentence, í expresses location, so it requires the dative, giving í rúminu.

The nouns are:

• barnið ← from barn, which is neuter
• bangsinn ← from bangsi, which is masculine
• dúkkan ← from dúkka, which is feminine
• rúminu ← from rúm, which is neuter

Gender matters in Icelandic because it affects:

• the form of the definite article
• adjective endings
• some pronouns
• declension patterns

So learning the gender with each noun is very useful.

Not always. This sentence is fairly straightforward, but Icelandic word order has its own rules, especially the verb-second pattern in main clauses.

Here the order is:

• Barnið sefur betur = main clause
• þegar bangsinn og dúkkan eru í rúminu = subordinate clause

If you move the þegar clause to the front, the main clause changes word order:

• Þegar bangsinn og dúkkan eru í rúminu, sefur barnið betur.

Notice that after the fronted subordinate clause, Icelandic puts the verb sefur before the subject barnið. That is a classic Icelandic pattern.

So Icelandic can look English-like sometimes, but not always.

Yes. A very natural alternative is:

Þegar bangsinn og dúkkan eru í rúminu, sefur barnið betur.

This means the same thing.

The main difference is emphasis:

• Barnið sefur betur þegar ... starts with the main point: the child sleeps better.
• Þegar ... , sefur barnið betur starts with the situation: when the teddy bear and the doll are in the bed.

Both are correct.

Because Icelandic usually expresses the by attaching it to the noun itself.

In this sentence:

• barnið = the child
• bangsinn = the teddy bear
• dúkkan = the doll
• rúminu = the bed in the required case

So English uses a separate word, but Icelandic often uses a suffix instead.

There is also a separate definite form in Icelandic in some contexts, especially with adjectives, but in a simple sentence like this, the attached article is the normal pattern.

Yes. Bangsi is a common everyday word meaning teddy bear or stuffed bear, especially in child-related contexts.

So bangsinn og dúkkan sounds very natural for the teddy bear and the doll.

It is exactly the kind of vocabulary you would expect in a sentence about a child sleeping.

The sentence has two parts:

1. Main clause
   Barnið sefur betur
   = The child sleeps better

2. Subordinate clause introduced by þegar
   þegar bangsinn og dúkkan eru í rúminu
   = when the teddy bear and the doll are in the bed

So the full structure is:

[main clause] + [when-clause]

This is a very useful pattern in Icelandic:

• X gerist þegar Y ...
• X happens when Y ...

It is a good sentence for practicing:

• definite nouns
• present tense verbs
• comparison with betur
• plural verb agreement
• case after í