Sautján nemendur komu í kennslustundina, en átján bækur voru á borðinu.

Because in Icelandic, numbers above one normally go with a plural noun.

So:

• sautján nemendur = seventeen students
• átján bækur = eighteen books

Here those noun phrases are also the subjects of the clauses, so the nouns appear in the nominative plural.

A useful comparison:

• einn nemandi = one student
• tveir nemendur = two students
• sautján nemendur = seventeen students

So nemandi becomes plural nemendur, and bók becomes plural bækur.

Usually no. In sentences like this, sautján and átján are normally treated as indeclinable for practical learner purposes, so the important inflection shows up on the noun, not on the numeral.

That is why you see:

• sautján nemendur
• átján bækur

rather than a changed form of the number itself.

This is different from lower numbers such as einn, tveir/tvær/tvö, þrír/þrjár/þrjú, and fjórir/fjórar/fjögur, which do change more visibly.

Nemendur is the nominative plural form of nemandi.

This looks irregular at first, but it follows a common pattern for many nouns ending in -andi:

• singular: nemandi
• plural: nemendur

So this is not a random one-off form, even though it has to be learned.

Yes, this is one of those Icelandic plurals that learners simply have to get used to.

The dictionary form is:

• bók = book

Its nominative plural is:

• bækur = books

So the plural involves both:

• a different ending
• a vowel change in the stem

This kind of stem change is common in Icelandic. It is best to learn nouns together with their plural forms whenever possible.

Because Icelandic usually adds the definite article onto the end of the noun instead of writing a separate word like English the.

So:

• kennslustund = lesson / class session
• kennslustundin = the lesson
• kennslustundina = the lesson (accusative singular)

In other words, -ina here contains the definite article plus the case ending needed by the sentence.

This is a very important Icelandic pattern.

Many prepositions, including í and á, use:

• accusative for motion/direction
• dative for location/static position

So here:

• í kennslustundina uses accusative because the idea is coming into / to the lesson
• á borðinu uses dative because the books are located on the table

Compare:

• fara í húsið = go into the house
• vera í húsinu = be in the house
• leggja bókina á borðið = put the book onto the table
• bókin er á borðinu = the book is on the table

In this sentence:

• nemendur = nominative plural
  It is the subject of komu.

• kennslustundina = accusative singular definite
  It follows í with a sense of motion/direction.

• bækur = nominative plural
  It is the subject of voru.

• borðinu = dative singular definite
  It follows á with a sense of location.

So this sentence is a good example of how Icelandic case is controlled both by sentence role and by prepositions.

Because both subjects are plural:

• sautján nemendur
• átján bækur

So the verbs appear in past tense plural:

• komu = came
• voru = were

Their dictionary forms are:

• koma = to come
• vera = to be

A simple contrast:

• nemandinn kom = the student came
• nemendurnir komu = the students came

and

• bókin var = the book was
• bækurnar voru = the books were

Because Icelandic does not have an indefinite article like English a/an, and its definite article is usually attached to the noun.

So:

• nemendur = students
• bækur = books

If you wanted the students or the books, you would normally say:

• nemendurnir
• bækurnar

So the sentence uses indefinite plural nouns for nemendur and bækur, but definite forms for kennslustundina and borðinu.

En usually means but, and sometimes while or a mildly contrastive and depending on context.

In this sentence, it links two clauses and shows a contrast or simple comparison between them:

• one clause about the students
• one clause about the books

So but is a very natural translation here.

Yes. This is completely normal.

Both clauses begin with the subject, so the order is:

• subject + verb + rest

That looks quite English-like:

• Sautján nemendur komu ...
• átján bækur voru ...

But Icelandic is a verb-second language in main clauses. That means if you move another element to the front, the verb still stays in second position.

For example:

• Í kennslustundina komu sautján nemendur.
• Á borðinu voru átján bækur.

So the sentence you were given is normal, but Icelandic also allows more word-order flexibility than English.

The dictionary forms are:

• nemandi = student
• kennslustund = lesson / class session
• bók = book
• borð = table

This matters because Icelandic words often look quite different once they are inflected:

• nemandi → nemendur
• bók → bækur
• kennslustund → kennslustundina
• borð → borðinu

When learning Icelandic, it is very useful to identify the dictionary form behind each inflected form you see.