Learn Hardcore Icelandic: Barnið finnur uppáhalds leikfangið sitt undir sófanum. - The child finds its favorite toy under the sofa.

Barnið – the child (nominative, singular, definite)
finnur – finds (3rd person singular present of að finna “to find”)
uppáhalds – favourite (literally “of-favourite”; used like an adjective)
leikfangið – the toy (accusative, singular, definite of leikfang, “toy”)
sitt – its/own (reflexive possessive pronoun referring back to barnið)
undir – under (preposition)
sófanum – the sofa (dative, singular, definite of sófi, “sofa”)

So the structure is:
Barnið (subject) finnur (verb) uppáhalds leikfangið sitt (object) undir sófanum (place).

Why is it Barnið and not barn? Where does the -ið come from?

Barn means child in general. Icelandic usually marks definiteness on the noun with a suffix instead of a separate word like the.

barn – a child (indefinite)
barnið – the child (definite)

-ið is the definite article suffix for neuter singular nouns in the nominative (and accusative).
So:

Nominative: barn – barnið (a child – the child)
Accusative: barn – barnið
Dative: barni – barninu
Genitive: barns – barnsins

Here barnið is the subject (nominative definite), so we use that form.

What form is finnur, and what is the basic pattern for the verb að finna?

Finnur is:

3rd person singular
present tense
indicative mood

of the verb að finna (to find).

Basic present‑tense forms:

ég finn – I find
þú finnur – you (sg.) find
hann / hún / það finnur – he / she / it finds
við finnum – we find
þið finnið – you (pl.) find
þeir / þær / þau finna – they find

So in Barnið finnur…, barnið is 3rd person singular, so you use finnur.

What exactly is uppáhalds? Why does it end in -s, and does it change its form?

Uppáhalds comes from the noun uppáhald (favourite, favourite thing). In front of another noun it appears in the genitive singular, uppáhalds, and functions like an (almost) indeclinable adjective:

uppáhalds leikfang – a favourite toy
uppáhalds leikfangið – the favourite toy

Points to note:

The -s is the normal genitive‑singular ending of uppáhald.
In this adjectival use (uppáhalds
- noun), it generally stays uppáhalds for all genders, numbers, and cases. Learners can treat it as “not changing”.
It usually comes right before the noun it describes.

So uppáhalds leikfangið = the favourite toy.

Why is leikfangið in that form? What case is it, and how does leikfang decline in the singular?

Leikfang is a neuter noun meaning toy. In the sentence, leikfangið is:

accusative (it is the direct object of finnur)
singular
definite (the toy)

For neuter nouns like leikfang, nominative and accusative usually look the same.

Singular of leikfang:

Nominative: leikfang – a toy / leikfangið – the toy
Accusative: leikfang – a toy / leikfangið – the toy
Dative: leikfangi – a toy / leikfanginu – the toy
Genitive: leikfangs – of a toy / leikfangsins – of the toy

Because Barnið finnur hvað? (finds what?) → uppáhalds leikfangið sitt, the noun is accusative, so leikfangið is the correct definite form.

Why do we use sitt and not sína or sinn for “its” here?

Sinn / sín / sitt is the reflexive possessive pronoun (his/her/its own), used when the owner is the subject of the clause.

It agrees in gender, number and case with the thing owned, not with the owner.

Here:

Owner: barnið (neuter) – but this does not control the form of sinn.
Owned thing: leikfangið – neuter, singular, accusative.

So we need the neuter singular accusative form of sinn:

Masculine: sinn (nom / acc sg)
Feminine: sín (nom / acc sg)
Neuter: sitt (nom / acc sg)

Since leikfangið is neuter accusative singular, we choose sitt.

If the object were feminine, e.g. bók (book), you’d say:

Barnið finnur uppáhalds bókina sína. – The child finds its favourite book.

There bókin(a) is feminine accusative singular, so you use sína.

Why is it undir sófanum and not undir sófann? How does the preposition undir work with cases?

Undir is a so‑called “two‑way” preposition in Icelandic. It can take:

dative – when describing location (where something is)
accusative – when describing motion/direction (to where something moves)

In the sentence:

The toy is simply located under the sofa. No movement into that position is expressed.
So we use dative: undir sófanum = under the sofa (at that place).

Contrast:

Barnið setur leikfangið undir sófann.
– The child puts the toy under the sofa (movement into that position → accusative sófann).

Here we have:

sófann – accusative singular definite (motion to under the sofa)
sófanum – dative singular definite (location under the sofa)

In Barnið finnur uppáhalds leikfangið sitt undir sófanum, we talk about where it is found, so we use the dative: sófanum.

What form is sófanum, and how does sófi decline in the singular?

Sófi is a masculine noun: sofa, couch.

In the sentence, sófanum is:

masculine
singular
dative
definite (with the -num ending)

Singular of sófi:

Nominative: sófi – a sofa / sófinn – the sofa
Accusative: sófa – a sofa / sófann – the sofa
Dative: sófa – a sofa / sófanum – the sofa
Genitive: sófa – of a sofa / sófans – of the sofa

Because undir takes the dative for location, we get undir sófanum.

Can the word order uppáhalds leikfangið sitt be changed? For example, can I say Barnið finnur sitt uppáhalds leikfang undir sófanum?

Yes, you can change the order a bit, and both versions are natural:

Barnið finnur uppáhalds leikfangið sitt undir sófanum.
Barnið finnur sitt uppáhalds leikfang undir sófanum.

Differences:

In (1), the focus feels slightly more on that specific toy, already known in the context:
- literally: the favourite toy its (own).
In (2), sitt is closer to English its own favourite toy, and some speakers feel a tiny bit more emphasis on the “ownness” / possession.

Both are grammatically correct and very common.
General rule: adjectives and possessives usually come before the noun, and their internal order can vary, with small differences in emphasis rather than correctness.

Could I leave out sitt and just say Barnið finnur uppáhalds leikfangið undir sófanum? Would that still make sense?

Yes, that sentence is still grammatically correct:

Barnið finnur uppáhalds leikfangið undir sófanum.
– The child finds the favourite toy under the sofa.

But:

Without sitt, it no longer explicitly says whose favourite toy it is.
It could be the favourite toy of someone else already known in the context (for example, the family’s favourite toy, or another child’s favourite toy).

With sitt, you clearly say it is the subject’s own toy:

Barnið finnur uppáhalds leikfangið sitt… = The child finds its own favourite toy…

So sitt adds the information “belonging to the subject”.

I thought Icelandic likes to glue words together. Why is it uppáhalds leikfangið and not one long word?

You will indeed often see this written as one compound:

uppáhaldsleikfangið – literally favourite-toy-the

In practice:

uppáhalds leikfangið (two words) is very common in everyday writing.
uppáhaldsleikfangið (compound) follows the general tendency of Icelandic to form compound nouns and is also correct.

Structurally, it is:

uppáhald (noun “favourite”) → genitive uppáhalds
- leikfang (noun “toy”)
  → uppáhaldsleikfang (“favourite toy”) + -ið (definite).

So you may encounter both spellings; as a learner, you should recognize that uppáhalds leikfangið and uppáhaldsleikfangið mean the same thing in this context.

How would the sentence change if the subject were plural, “The children find their favourite toy under the sofa”?

You would change the subject, the verb, and possibly the object, depending on what you mean:

If all the children together find one favourite toy (shared):

Börnin finna uppáhalds leikfangið sitt undir sófanum.
- Börnin – the children (plural)
- finna – 3rd person plural present of að finna
- uppáhalds leikfangið sitt – their (own) favourite toy (one toy, owned by them collectively)

If each child finds their own favourite toy (many toys):

Börnin finna uppáhalds leikföngin sín undir sófanum.
- leikföngin – the toys (plural of leikfang)
- sín – reflexive possessive agreeing with plural leikföngin

Remember: sinn/sín/sitt always agrees with the thing owned, so:

sitt – neuter singular (one toy)
sín – neuter plural (many toys)

In both cases, undir sófanum stays the same.