Þessi bók er fyrir fullorðna, ekki fyrir lítil börn.

Because þessi has to agree with bók in gender, number, and case.

• bók = book
• It is feminine
• It is singular
• Here it is in the nominative case, because it is the subject of er (is)

So the correct form of this is:

• masculine singular: þessi
• feminine singular: þessi
• neuter singular: þetta

That means:

• þessi bók = this book
• but þetta barn = this child because barn is neuter

So þetta bók is wrong because bók is not neuter.

In Icelandic, when you use a demonstrative like þessi (this), the noun usually appears without the suffixed definite article.

So:

• bók = book
• bókin = the book
• þessi bók = this book

Using þessi bókin is generally not the normal standard way to say this book in a simple sentence like this.

A good way to think about it is:

• English: this book
• Icelandic: þessi bók

not

• this the book

Fullorðinn is the dictionary form, but Icelandic words change form depending on grammar.

In this sentence, fullorðna means adults and is being used almost like a noun, even though it is originally an adjective meaning grown-up / adult.

The basic idea is:

• fullorðinn = adult, grown-up (singular dictionary form)
• fullorðna = adults, in the form required here

So in fyrir fullorðna, the word means for adults.

This is very common in Icelandic: adjectives can be used substantively, meaning they stand in for a noun that is understood.

Because the preposition fyrir here takes the accusative case when it means for in the sense of intended audience or benefit.

So after fyrir, you need the accusative:

• fyrir fullorðna = for adults

Not:

• fyrir fullorðnir — wrong here, because that is not the accusative form
• fyrir fullorðnum — also wrong here in this meaning

This is one of the big things Icelandic learners have to get used to: prepositions often control a specific case.

In this sentence:

• fyrir + accusative = for
• fullorðna is the accusative plural form

The adjective lítill (small/little) has to agree with börn.

Here:

• börn = children
• it is neuter plural
• after fyrir, it is in the accusative
• for neuter plural, nominative and accusative are the same

So the adjective must also be in the matching form:

• lítil börn = small children

Why not the others?

• litil is just misspelled; the correct form is lítil
• litlu börn does not match this noun phrase here

A helpful comparison:

• lítið barn = a small child
• lítil börn = small children

Because the sentence is talking about children in the plural, not a child in the singular.

• barn = child
• börn = children

This is an irregular plural, so it does not just add a simple ending the way some nouns do.

So:

• fyrir lítið barn = for a small child
• fyrir lítil börn = for small children

Yes, that is a good way to feel the word.

The adjective fullorðinn literally has the sense of being fully grown / grown-up, and when used substantively in the plural, fullorðna means adults or grown-ups.

So all of these are close in meaning:

• fyrir fullorðna = for adults
• for grown-ups

The exact English choice depends on tone, but adults is usually the most natural translation.

In this sentence, repeating fyrir is the natural and clear way to say it:

• fyrir fullorðna, ekki fyrir lítil börn
• for adults, not for small children

Could you leave out the second fyrir? In some languages that kind of omission is easy, but in Icelandic it is much more natural here to repeat it.

Repeating the preposition makes the contrast very clear:

• for adults
• not for small children

So learners should treat the full version with repeated fyrir as the standard pattern.

Ekki means not, and here it negates the second prepositional phrase:

• ekki fyrir lítil börn = not for small children

So the structure is:

• Þessi bók er fyrir fullorðna = This book is for adults
• ekki fyrir lítil börn = not for small children

It does not mean the whole sentence is negative. The book is for someone; it is just not for small children.

Lítil börn is indefinite: small children.

• börn = children
• börnin = the children

So:

• fyrir lítil börn = for small children
• fyrir litlu börnin = for the small children

Notice that if you make the noun definite, the adjective changes too:

• indefinite: lítil börn
• definite: litlu börnin

That is because Icelandic adjectives follow different declension patterns depending on whether the noun phrase is definite or indefinite.

Because they involve different kinds of agreement and different words.

1. Þessi is a demonstrative word meaning this
2. lítil is an adjective meaning small

Both must agree with their nouns, but they belong to different word classes and have their own forms.

Also, the nouns they modify are different:

• bók = feminine singular
• börn = neuter plural

So naturally the forms are different:

• þessi bók = this book
• lítil börn = small children

The structure is:

• Þessi bók — subject: This book
• er — verb: is
• fyrir fullorðna — complement: for adults
• ekki fyrir lítil börn — contrasting negative phrase: not for small children

So literally:

• This book is for adults, not for small children.

It is a very normal Icelandic pattern:

[subject] + er + fyrir + [group] + ekki fyrir + [other group]

A natural singular version would be:

• Þessi bók er fyrir fullorðinn, ekki fyrir lítið barn.

The key changes are:

• fullorðna → fullorðinn for singular adult
• lítil börn → lítið barn for singular small child

So the original sentence is plural:

• fyrir fullorðna = for adults
• fyrir lítil börn = for small children

And the singular version is:

• fyrir fullorðinn = for an adult
• fyrir lítið barn = for a small child

A rough English-friendly pronunciation is:

• Þessi bók er fyrir fullorðna, ekki fyrir lítil börn
• THESS-ih bohk er FEER-ir FUTL-orth-na, EHK-ih FEER-ir LEE-til burn

A few useful pronunciation notes:

• Þ / þ is like th in thin
• ð is like th in this, though in some positions it can sound weaker
• ó is like a long o
• ö in börn is a rounded vowel somewhat like the vowel in French peur or German hören; English does not have an exact match

If pronunciation matters for your learning, the trickiest parts here are usually:

• þessi
• fullorðna
• börn