Kaupin voru góð.

Kaupin is the definite nominative plural form of kaup.

A few useful points:

• kaup can mean purchase, deal, bargain, or in some contexts wages/pay
• the ending -in is the attached definite article, so kaupin means the purchase(s)/the deal/the bargain, depending on context
• in this sentence, it is the subject

So grammatically, kaupin is not just a basic dictionary form plus a separate word for the. In Icelandic, the is usually added onto the end of the noun.

This is a very common learner question. Some Icelandic nouns are often used in the plural where English may prefer a singular expression.

With kaup, Icelandic often treats the idea as plural in form, especially in expressions referring to a purchase, deal, or bargain. So even if English says something like The deal was good, Icelandic may use a plural-looking noun.

That is why the verb is also plural:

• kaupin = plural subject
• voru = were, not was

So the plural is not a mistake; it is just how Icelandic commonly expresses this idea.

Voru is the past tense, 3rd person plural form of vera (to be).

Here is the relevant part of the verb:

• er = is
• eru = are
• var = was
• voru = were

Because kaupin is treated as plural, Icelandic uses voru.

So:

• Kaupin voru ... = The purchase(s)/deal were ...

Because Icelandic adjectives must agree with the noun they describe or refer back to in:

• gender
• number
• case

Here, kaupin is neuter plural nominative, so the adjective must also be neuter plural nominative.

That gives:

• masculine singular: góður
• feminine singular: góð
• neuter singular: gott
• neuter plural: góð

So góð here is the correct agreement form for kaupin.

It is working predicatively, not attributively.

In other words:

• góð kaup = good purchases / a good deal
  
  • here the adjective comes directly with the noun
• kaupin voru góð = the purchases/deal were good
  
  • here the adjective comes after to be and describes the subject through the verb

So in this sentence, góð is a predicate adjective.

Yes. Kaupin is in the nominative case, because it is the subject of the sentence.

The adjective góð also appears in the matching form. In Icelandic, predicate adjectives normally agree with the subject in:

• case
• number
• gender

So here the pattern is:

• kaupin = nominative, neuter, plural
• góð = nominative, neuter, plural

This agreement is one of the most important things to notice in Icelandic sentences.

Because Icelandic usually expresses the as a suffix attached to the noun.

So instead of having a separate article like English the, Icelandic often does this:

• kaup = purchase(s), bargain
• kaupin = the purchase(s), the bargain

The -in ending is what makes it definite here.

Yes, but the normal basic order here is:

• Kaupin — subject
• voru — verb
• góð — complement/adjective

So:

• Kaupin voru góð.

Because Icelandic is a verb-second (V2) language, other word orders are possible if something else comes first. For example, if you moved another element to the front, the verb would still usually stay in second position.

But as a simple standalone sentence, Kaupin voru góð is the most natural neutral order.

A rough learner-friendly pronunciation is something like goth or goath, but not exactly like either English word.

A few important details:

• ð in Icelandic is often like the th in this
• the vowel is longer/rounder than English o in many dialects and contexts
• the whole word is one syllable

So góð is approximately gohth.

If you are learning pronunciation, the main thing is: do not pronounce ð like a hard d.

Yes. Kaup can have a few related meanings depending on context, such as:

• purchase
• deal
• bargain
• wages/pay

So the exact natural English translation depends on the situation.

In this sentence, since the meaning has already been given to you, the important grammar point is that the noun is treated as plural definite and controls plural agreement:

• kaupin → voru → góð

That agreement pattern is the key thing to learn from the sentence.