Barnið hljóp grátandi til móður sinnar þegar það fann ekki leikfangið sitt.

Because -ið is the definite article attached to the noun.

• barn = child
• barnið = the child

Icelandic usually adds the to the end of the noun instead of using a separate word like English does.

Also, barn is a neuter noun, and barnið is the singular definite nominative/accusative form.

Hljóp is the past tense of the verb hlaupa (to run).

So:

• hlaupa = to run
• hljóp = ran

It is a strong verb, so the vowel changes in the past tense instead of just adding an ending.

Grátandi comes from gráta (to cry). It is a present participle, and here it means crying or while crying.

So hljóp grátandi means something like:

• ran crying
• ran while crying

In this sentence, it describes how the child ran.

There are two important things happening here:

1. til requires the genitive case
2. móðir changes to móður in the genitive singular

So:

• dictionary form: móðir = mother
• after til: móður

Then sinnar is the reflexive possessive, also in the form required here.

So:

• til móður sinnar = to its own mother

Because sinn / sín / sitt changes form to match the thing possessed, not the possessor.

In this sentence:

• móður sinnar: the possessed noun is móður, which is feminine singular genitive, so we get sinnar
• leikfangið sitt: the possessed noun is leikfangið, which is neuter singular, so we get sitt

This is one of the most important things to remember about sinn in Icelandic: it agrees with the noun being owned.

It is the reflexive possessive: his/her/its/their own, when it refers back to the subject of the clause.

In this sentence:

• Barnið ... til móður sinnar = the child ran to its own mother
• það fann ekki leikfangið sitt = it did not find its own toy

If you used a non-reflexive possessive instead, the meaning could change and suggest someone else’s mother or someone else’s toy.

Because barn is a neuter noun, and Icelandic pronouns usually follow the grammatical gender of the noun.

So:

• barnið = the child
• það = it

Even though in English we often say he, she, or sometimes they, Icelandic commonly uses það when referring back to barn.

Because ekki (not) normally comes after the finite verb in a neutral clause.

So:

• það fann ekki leikfangið sitt = it did not find its toy

This word order is very normal in Icelandic.

It is the direct object of fann (found), so it is in the accusative.

The noun is leikfang (toy), which is neuter. In the singular definite, the nominative and accusative look the same:

• leikfangið = the toy

So you know it is accusative here because of its job in the sentence, not because the form looks different.

Because leikfangið sitt means its toy / the toy that belongs to it, with a definite noun.

• leikfang = toy
• leikfangið = the toy

Using sitt leikfang would sound more like its own toy / a toy of its own, often less definite or with a slightly different emphasis.

So in this sentence, leikfangið sitt is the natural way to say its toy.

Þegar means when.

It introduces the time clause:

• þegar það fann ekki leikfangið sitt = when it did not find its toy

So the whole sentence structure is:

• main clause: Barnið hljóp grátandi til móður sinnar
• subordinate clause: þegar það fann ekki leikfangið sitt

Yes. After þegar, you get a subordinate clause, and the word order is the normal subordinate-clause pattern:

• þegar það fann ekki leikfangið sitt

That is:

• þegar = conjunction
• það = subject
• fann = finite verb
• ekki = negation
• leikfangið sitt = object

This is a very common Icelandic sentence pattern.

Here are the most useful ones:

• barnið → barn = child
• hljóp → hlaupa = to run
• grátandi → gráta = to cry
• móður → móðir = mother
• fann → finna = to find
• leikfangið → leikfang = toy

This is helpful because Icelandic words often appear in forms that look quite different from the dictionary form.