Að klára alla bókina í kvöld væri óraunhæft, en það er raunhæft að lesa tvo kafla.

Because að + infinitive can function as a whole clause in Icelandic, much like to finish in English.

So:

• Að klára alla bókina í kvöld = To finish the whole book tonight

This entire infinitive phrase is acting as the subject of the sentence. Icelandic does this very naturally.

So the structure is roughly:

• Að klára alla bókina í kvöld = subject
• væri óraunhæft = would be unrealistic

Væri is a form of vera (to be) used for hypothetical, tentative, or conditional-style statements. Here it gives the sense of:

• would be unrealistic

instead of the more direct:

• is unrealistic

So:

• Að klára alla bókina í kvöld væri óraunhæft = finishing the whole book tonight would be unrealistic

This sounds a little softer or more evaluative than er óraunhæft.

You could also say:

• Að klára alla bókina í kvöld er óraunhæft

That would be correct too, but it sounds more direct and definite.

This creates a nice contrast:

• first: one option is presented as hypothetical or unrealistic
• second: another option is presented as a real, practical possibility

So the meaning is roughly:

• Finishing the whole book tonight would be unrealistic, but reading two chapters is realistic.

The first part judges an over-ambitious idea.
The second part states a more practical fact.

You could make both parts use væri, but the original version sounds very natural.

Because they are neuter singular forms.

In Icelandic, when an adjective refers to:

• an infinitive clause, or
• a whole idea, or
• a dummy subject like það

it is very often put in the neuter singular.

So:

• Að klára alla bókina í kvöld is treated as a neuter singular idea
• therefore: óraunhæft

And in:

• það er raunhæft að lesa tvo kafla

the það is also neuter singular, so you get:

• raunhæft

They mean:

• raunhæft = realistic, feasible
• óraunhæft = unrealistic, not feasible

The prefix ó- often negates a word, similar to un- in English.

So:

• raunhæfur = realistic
• óraunhæfur = unrealistic

In this sentence, you see the neuter singular forms:

• raunhæft
• óraunhæft

Because klára takes a direct object, and that object is in the accusative case.

Here the object is the whole book:

• bókin = the book
• bókina = the book (accusative)

And allur (all / whole) has to agree with the noun in gender, number, and case:

• bók is feminine singular
• the phrase is accusative singular
• so allur becomes alla

That gives:

• alla bókina = the whole book

So this is not random memorization: both words are showing the grammar of the object.

Because í kvöld is a standard Icelandic time expression meaning tonight / this evening.

This is one of those phrases you should learn as a chunk. Icelandic prepositions often do not match English word-for-word.

So just remember:

• í kvöld = tonight
• í morgun = this morning
• í dag = today

Trying to translate the preposition literally is usually not helpful here.

Here það is a dummy subject, similar to English it in:

• It is realistic to read two chapters

The real content comes later:

• að lesa tvo kafla = to read two chapters

So the structure is like:

• það er raunhæft = it is realistic
• að lesa tvo kafla = to read two chapters

This is a very common pattern in Icelandic.

Yes, but it would sound more formal or marked.

For example:

• Raunhæft er að lesa tvo kafla

This is grammatical, but less neutral than:

• Það er raunhæft að lesa tvo kafla

The version with það is the most natural everyday structure for many contexts.

Because tvo kafla is the accusative masculine plural, and it is the object of lesa.

• kafli is masculine
• lesa takes a direct object
• so the phrase must be in the accusative

The numeral two changes form depending on gender and case.

For masculine:

• nominative: tveir
• accusative: tvo

And the noun also changes:

• nominative plural: kaflar
• accusative plural: kafla

So:

• tveir kaflar = two chapters as a subject
• lesa tvo kafla = read two chapters as an object

Yes, absolutely. That would be a very natural sentence too.

Compare:

• Að klára alla bókina í kvöld væri óraunhæft
• Það væri óraunhæft að klára alla bókina í kvöld

Both mean essentially the same thing.

The difference is mainly one of structure and emphasis:

• starting with Að klára... puts emphasis on the action itself
• using Það væri... að klára... delays the action phrase and can sound a bit smoother in conversation

Both patterns are common in Icelandic.

Yes. Klára is a very common everyday verb meaning to finish, complete, or get done with something.

So:

• klára bókina = finish the book
• klára verkefnið = finish the assignment

It is very natural in modern Icelandic.

Yes: it combines two very common Icelandic patterns.

1. Infinitive clause as subject

   • Að klára alla bókina í kvöld væri óraunhæft

2. Dummy subject + adjective + infinitive clause

   • það er raunhæft að lesa tvo kafla

So this one sentence is useful because it shows two ways Icelandic can talk about actions as ideas:

• To do X would be unrealistic
• It is realistic to do Y

Those are both very common and worth learning as sentence patterns.