Í ísskápnum er líka dós af köldu gosi fyrir gestina.

This is a very common Icelandic word-order pattern.

Icelandic is a verb-second language. That means the finite verb usually comes in the second position in the clause. Here:

• Í ísskápnum = first element
• er = second element
• dós af köldu gosi = the subject comes after the verb

So the sentence structure is completely normal.

Starting with Í ísskápnum puts focus on the location: In the fridge, there is also a can of cold soda...

You could also say Það er líka dós af köldu gosi í ísskápnum fyrir gestina, but the original sentence is very natural and slightly more location-focused.

Because í takes different cases depending on meaning.

Here it means in in the sense of location, so it takes the dative. That gives:

• dictionary form: ísskápur = fridge
• dative definite form: ísskápnum = in the fridge

So:

• í ísskápnum = in the fridge

A very useful contrast is:

• í ísskápnum = in the fridge, already there, location
• í ísskápinn = into the fridge, motion/direction

It shows that the noun is both:

• definite: the fridge
• in the dative singular

Icelandic usually puts the definite article on the end of the noun instead of using a separate word like English the.

So ísskápnum means roughly the fridge in the form required after í for location.

Because Icelandic has no indefinite article.

So:

• dós can mean a can
• maður can mean a man
• hús can mean a house

Icelandic does have a definite article, but it is usually attached to the noun:

• dós = a can
• dósin = the can

So dós af köldu gosi simply means a can of cold soda.

Dós is in the nominative singular because it is the grammatical subject of the sentence.

Even though English often uses there is, Icelandic still has a normal subject here:

• er = is
• dós af köldu gosi = a can of cold soda

So the basic idea is: A can of cold soda is also in the fridge for the guests, even though the Icelandic sentence begins with the location phrase.

Because the preposition af takes the dative.

The noun gos is neuter, so in the dative singular it becomes:

• gos → gosi

The adjective has to agree with it in case, number, and gender:

• kaldur → köldu in neuter singular dative

So:

• af köldu gosi = of cold soda

This is a very common pattern after af.

Here af expresses the contents of the can: a can of cold soda.

This is similar to English of in phrases like:

• a cup of coffee
• a glass of water
• a can of soda

So dós af köldu gosi literally means a can of cold soda.

In Icelandic, gos is usually treated as a mass noun in this kind of phrase, like soda or soft drink in general.

So after a container word, Icelandic normally says:

• dós af gosi = a can of soda
• glas af vatni = a glass of water

That is why gosi is singular here.

Because fyrir here means for in the sense of intended for or meant for, and in this use it takes the accusative.

So:

• gestir = guests
• gestina = the guests, accusative plural definite

Therefore:

• fyrir gestina = for the guests

This means the soda is there for the guests to have.

Because they play different roles in the sentence.

• ísskápnum = the fridge
  This is a specific, known fridge.

• gestina = the guests
  These are specific guests already known from the context.

• dós = a can
  This is being introduced as a new item, so it is indefinite.

This is very natural in Icelandic: known/specific things are often definite, while newly mentioned things are often indefinite.

Líka means also or too.

In this sentence it adds the meaning that this is one more thing in the situation already being talked about: there is also a can of cold soda.

Its placement is natural because Icelandic often puts adverbs like líka after the finite verb:

• Í ísskápnum er líka dós...

That sounds smooth and idiomatic. The exact placement of líka can vary a little depending on emphasis, but here it simply means also for the whole statement.