Við borðuðum lítinn forrétt fyrst og deildum svo aðalréttinum saman.

They are both past tense, 1st person plural verb forms, so they mean we ate and we shared.

• borða = to eat
  → við borðuðum = we ate
• deila = to share / divide
  → við deildum = we shared

In Icelandic, the verb ending often shows who is doing the action, so -ðum here tells you it is we in the past.

Because the subject is the same in both parts of the sentence: we did both actions.

So Icelandic can say:

• Við borðuðum ... og deildum ...

instead of repeating:

• Við borðuðum ... og við deildum ...

Both are understandable, but leaving out the second við is very natural when the same subject continues.

Because lítinn forrétt is the direct object of borðuðum.

The verb borða takes an object in the accusative case, so both the adjective and the noun change form:

• nominative: lítill forréttur = a small starter
• accusative: lítinn forrétt = a small starter (as the thing being eaten)

So after borðuðum, you need the accusative:

• Við borðuðum lítinn forrétt.

Because lítinn agrees with forrétt in:

• gender: masculine
• number: singular
• case: accusative
• definiteness: indefinite

The noun forréttur is masculine singular, and here it is in the accusative. The adjective has to match it, so:

• lítill forréttur = nominative
• lítinn forrétt = accusative

This agreement between adjective and noun is a very important feature of Icelandic grammar.

Because deila normally takes the dative case for the thing being shared.

So:

• aðalréttur = nominative
• aðalréttinum = dative singular definite

That is why Icelandic says:

• deila aðalréttinum

rather than using the accusative.

This is something English speakers often have to memorize: different Icelandic verbs require different cases.

Because the sentence treats them differently:

• lítinn forrétt = a small starter
  This introduces it as an indefinite item.
• aðalréttinum = the main course
  This refers to a specific main course.

In Icelandic, definiteness is usually shown by adding the definite article to the end of the noun:

• aðalréttur = a main course
• aðalrétturinn = the main course
• aðalréttinum = to/from/on the main course, or here the dative form after deila

So the noun is both definite and dative here.

Fyrst means first.

It tells you the order of actions:

• Við borðuðum lítinn forrétt fyrst = We ate a small starter first

In this sentence it comes after the object, which is perfectly natural in Icelandic. Icelandic word order can be flexible, but this placement sounds normal and clear.

Svo here means then, after that, or next.

It connects the two actions in time:

• first they ate a starter
• svo they shared the main course

So svo is marking the next step in the sequence.

Because Icelandic often keeps the finite verb very early in the clause.

After og, the sentence continues directly with the verb:

• og deildum svo aðalréttinum saman

Since the subject við is omitted in the second clause, the verb naturally comes first there.

You could also make other word orders in different contexts, such as:

• og svo deildum við aðalréttinum saman

But the version in your sentence is very natural and compact.

Saman means together, and it adds the idea that the two people shared it with each other.

So:

• deildum aðalréttinum = shared the main course
• deildum aðalréttinum saman = shared the main course together

In English, shared already often implies this, but Icelandic commonly uses saman to make the idea explicit and natural.

Here are the main base forms:

• við = we
• borðuðum → borða = to eat
• lítinn → lítill = small
• forrétt → forréttur = starter, appetizer
• fyrst = first
• deildum → deila = to share, divide
• svo = then
• aðalréttinum → aðalréttur = main course
• saman = together

This is useful because Icelandic words often appear in changed forms depending on case, number, tense, and sentence role.

More or less, yes:

• Við = we
• borðuðum = ate
• lítinn forrétt = a small starter
• fyrst = first
• og = and
• deildum = shared
• svo = then
• aðalréttinum = the main course
• saman = together

A very literal English version would be:

• We ate a small starter first and shared the main course together then.

But more natural English would usually be:

• We first ate a small starter and then shared the main course.

So the Icelandic word order and the most natural English word order do not have to match exactly.