Söluborðið er fullt.

It is a compound noun:

• sölu- = sale / selling
• borð = table, counter
• -ið = the (the neuter singular definite ending)

So söluborðið means something like the sales table, the sales counter, or the stall/counter for selling depending on context.

Icelandic very often builds words this way, by combining smaller words into one compound.

In Icelandic, the definite article is usually a suffix, not a separate word like English the.

So:

• söluborð = sales counter / a sales counter
• söluborðið = the sales counter

This is very normal in Icelandic. English speakers often need time to get used to the idea that the is built into the noun.

Er is the present tense, third-person singular form of að vera = to be.

So:

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

Since söluborðið is a singular noun, Icelandic uses er:
Söluborðið er fullt. = The sales counter is full.

Because the adjective has to agree with the noun in gender, number, and usually case.

The noun söluborðið is:

• neuter
• singular
• nominative here

So the adjective must also be neuter singular nominative:

• fullur = masculine
• full = feminine
• fullt = neuter

That is why the sentence uses fullt.

You learn the gender as part of the noun. The base noun borð is a neuter noun, and compounds built on it usually keep that gender.

A useful clue here is also the definite ending -ið, which is a common neuter singular definite ending.

So:

• borð = neuter
• söluborð = also neuter
• söluborðið = the neuter singular form

It is in the nominative case because it is the subject of the sentence.

This is a basic X is Y sentence:

• söluborðið = subject
• er = verb
• fullt = predicate adjective

With að vera in a sentence like this, the predicate adjective normally matches the subject, so fullt is nominative too.

It can mean several related things, just like English full.

With something like söluborðið, fullt could mean:

• full of items
• covered with things
• occupied
• packed

So the exact nuance depends on context. It does not have to mean full of liquid. It could simply mean the sales counter has no free space left.

This is the normal word order for a simple statement in Icelandic:

• subject
• verb
• complement

So:

• Söluborðið = subject
• er = verb
• fullt = complement

That gives the standard order: Söluborðið er fullt.

Icelandic can change word order in some situations, especially in questions or for emphasis, but this sentence is the plain neutral form.

Grammatically, söluborð without -ið is the indefinite form, so yes, it does not mean the sales counter.

However, Icelandic does not have a separate word for a/an, so bare nouns can feel more general or context-dependent than in English. In actual usage, whether it means a sales counter, sales counters in general, or something else depends on context.

In your sentence, söluborðið is clearly definite: the sales counter.

A rough guide:

• ö is somewhat like the vowel in French peur or German schön. For English speakers, there is no exact match, but it is not the same as o.
• ð is like the th in this, that, mother.

So söluborðið is roughly something like SUR-lu-bor-thith or SÖ-lu-bor-thith, but that is only approximate.

A few cautions:

• Icelandic pronunciation is not easy to capture with English spelling.
• The final -ið is not pronounced like English id.
• The stress is normally on the first syllable: SÖ-lu-borðið.

Not necessarily. Like many words involving borð, it can refer to more than just a literal table.

Depending on context, söluborð could mean:

• a sales table
• a sales counter
• a display table
• a stall or booth area for selling

So you should think of it as a practical selling surface or counter, not only a household table.

No. Here fullt is an adjective, not an adverb.

It describes the subject söluborðið, and it changes form to agree with that noun. That is what adjectives do in Icelandic.

An adverb would not agree with the noun in gender and number like this. The neuter form fullt here is specifically the adjective agreeing with a neuter singular noun.

Yes. That would mean The sales counter is full of goods/products.

The pattern fullur/full/fullt af ... is very common in Icelandic:

• fullt af fólki = full of people
• fullt af mat = full of food
• fullt af vörum = full of goods/products

So your original sentence can stand alone, but Icelandic also often adds af + dative noun to say what something is full of.