Barnið fann steininn.

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

So:

• barnið = the child
• steininn = the stone

This is called the suffixed definite article. English uses a separate word (the child), but Icelandic often builds that meaning into the noun itself.

Because the nouns belong to different grammatical genders and also take different article endings.

• barn is neuter
• steinn is masculine

When the definite article is added, the ending changes depending on gender and case. So:

• barn → barnið
• steinn → steininn

These are normal Icelandic patterns, not random changes.

Because steinn is the nominative singular form, but here the noun is the direct object, so it appears in the accusative singular.

The forms are:

• nominative: steinn = a stone
• accusative: stein = a stone (as an object)
• accusative definite: steininn = the stone

So the extra -n in steinn belongs to the nominative form. Once the noun moves into the accusative, the stem becomes stein-, and then the definite article is added: stein + inn = steininn.

Because it is the direct object of the verb fann (found).

In the sentence:

• Barnið = the subject, the one doing the action
• fann = found
• steininn = the object, the thing that was found

The verb finna normally takes a direct object in the accusative, so the stone becomes steininn.

Fann is the past tense of the verb finna (to find).

So:

• finna = to find
• finnur = finds
• fann = found

This is a strong verb, so the vowel changes in the past tense rather than just adding a simple ending.

Because Icelandic usually expresses the past tense directly in the verb itself.

English can say:

• The child found the stone
• or in questions/negatives use did

But Icelandic simply uses the past tense form:

• fann = found

So there is no extra word corresponding to English did in this sentence.

In this sentence, the main clue is word order:

• Barnið comes before the verb
• steininn comes after the verb

That matches the usual Icelandic subject–verb–object order.

Case also helps in many Icelandic sentences, but here barnið has the same form in nominative and accusative, so word order is especially useful. Meanwhile, steininn is clearly an accusative object form.

Barn is a neuter noun, and yes, that matters a lot in Icelandic.

Gender affects things like:

• the form of the definite article
• adjective endings
• pronouns that refer to the noun

That is why the child is barnið and not a masculine or feminine form.

It is singular: the child.

The base noun is barn = child, and barnið means the child.
If it were plural, the form would be different.

Yes. If you wanted to say A child found a stone, you would normally use the bare nouns:

• Barn fann stein.

So the difference is:

• Barnið fann steininn. = The child found the stone.
• Barn fann stein. = A child found a stone.

Icelandic often uses the noun without any article for an indefinite meaning.

The basic word order is:

• Barnið = subject
• fann = verb
• steininn = object

So it follows the common SVO pattern:

Subject – Verb – Object

That makes it very similar to ordinary English word order in a simple statement.

A careful approximate pronunciation is:

Barnið fann steininn
BAR-nith fann STAY-nihn

A few useful notes:

• ð in barnið is like the th in this, though in some positions it can be weaker
• ei in steininn sounds like ay in day
• stress usually falls on the first syllable of each word in Icelandic

So the main stresses are on BAR-, fann, and STEI-.