Bókin er um sálina.

Because -in at the end is the definite article in Icelandic.

• bók = a book / book
• bókin = the book

Unlike English, Icelandic usually attaches the to the end of the noun instead of writing it as a separate word.

So:

• Bókin = the book

Also notice that bók is a feminine noun, and its definite form in the nominative singular is bókin.

This is because the preposition um normally takes the accusative case.

The noun sál is feminine:

• sálin = the soul in the nominative
• sálina = the soul in the accusative

Since the sentence uses um (about), Icelandic requires the accusative form:

• um sálina = about the soul

So the ending changes because the noun is not in the basic subject form.

For standard Icelandic usage, um takes the accusative.

That means when you use um, the noun after it should be in the accusative:

• um bók / um bókina
• um sál / um sálina

This is an important thing to memorize with Icelandic prepositions: many of them control a particular case.

Er is the present tense, third person singular form of vera (to be).

So:

• ég er = I am
• þú ert = you are
• hann / hún / það er = he / she / it is

In Bókin er um sálina, the subject is Bókin (the book), which is third person singular, so er is the correct form.

Because Icelandic usually expresses definiteness with a suffix article attached to the noun.

Compare:

• English: the book
• Icelandic: bókin

And:

• English: the soul
• Icelandic: sálin / sálina depending on case

This is one of the most noticeable differences from English. Instead of putting the before the noun, Icelandic often adds an ending.

The dictionary form is sál.

If you look up the word in a dictionary, you would usually find:

• sál = soul

Then its forms include things like:

• sál = nominative/accusative indefinite singular
• sálar = genitive singular
• sálin = nominative definite singular
• sálina = accusative definite singular

So sálina is an inflected form of the noun sál.

This is a very normal Icelandic sentence pattern:

• Bókin = subject
• er = verb
• um sálina = prepositional phrase

So it follows a straightforward pattern similar to English:

• The book / is / about the soul

Icelandic can have more flexible word order than English in some contexts, but this sentence is basic and neutral.

A rough English-friendly guide would be:

• Bókin ≈ BOH-kin
• er ≈ ehr
• um ≈ um (with a short vowel, not like English yum)
• sálina ≈ SAU-lee-na

A few pronunciation notes:

• ó is a distinct vowel, roughly like o in go, though not exactly the same.
• á sounds roughly like ow in now.
• Stress in Icelandic usually falls on the first syllable: BÓ-kin, SÁ-li-na.

This is only an approximation, but it can help as a starting point.

Because the subject is already stated: Bókin (the book).

So Icelandic, like English, does not need an extra pronoun here. The sentence is directly:

• The book is about the soul

If you added a pronoun as well, it would be unnecessary in this context.

Yes. Um has several uses depending on context, though about is one of the most common.

It can also be used in meanings related to:

• around
• concerning
• about / regarding
• certain time or movement expressions in other contexts

But in Bókin er um sálina, the meaning is clearly about / concerning.

It is singular.

Here is the breakdown:

• sál = soul
• sálin = the soul
• sálina = the soul, in accusative singular

If it were plural, the form would be different. So this sentence is talking about the soul, not souls.