Frystirinn er fullur í dag, svo við kaupum ekki meiri mat.

Frystirinn means the freezer.

The base noun is frystir = freezer.
The ending -inn is the definite article, so Icelandic often says the by attaching it to the noun instead of putting a separate word in front.

So:

• frystir = a freezer / freezer
• frystirinn = the freezer

This is a very common feature of Icelandic nouns.

Because adjectives in Icelandic change form to match the noun they describe.

Here, frystirinn is:

• masculine
• singular
• nominative

So the adjective fullur has to appear in the matching masculine singular nominative form.

That is why you get:

• frystirinn er fullur = the freezer is full

If the noun were feminine or neuter, the adjective form would be different.

Í dag means today.

Literally, it comes from:

• í = in
• dag = a form of day

So historically it is something like in day, but in modern Icelandic it simply works as the fixed expression today.

There is no separate the here because Icelandic just uses this expression as it is: í dag.

Here svo means so or therefore.

It connects the two parts of the sentence:

• Frystirinn er fullur í dag = the freezer is full today
• svo við kaupum ekki meiri mat = so we are not buying more food

Be aware that svo can mean different things in other contexts, such as then or sometimes part of expressions with other meanings. But in this sentence it is simply a result word: so.

Because Icelandic verbs change form depending on the subject.

The verb is að kaupa = to buy.
Its present tense form with við = we is kaupum.

So:

• ég kaupi = I buy
• þú kaupir = you buy
• við kaupum = we buy

So við kaupum is the correct form for we buy / we are buying.

In a normal main clause, ekki usually comes after the finite verb.

So:

• við kaupum ekki = we do not buy / we are not buying

This word order is very typical in Icelandic.
English puts not after an auxiliary verb, but Icelandic often simply puts ekki after the conjugated verb.

Because Icelandic uses meiri when talking about a greater amount of something.

Here matur means food, and in this sentence it appears as mat because it is the object of the verb.

So:

• meiri mat = more food

This is about quantity/amount, not number.

A useful comparison:

• meiri matur / meiri mat = more food
• fleiri epli = more apples

Use meiri for more of a mass or amount, and fleiri for more individual countable things.

Because matur is the dictionary form, but nouns change form depending on their role in the sentence.

Here food is the direct object of kaupum = buy, so matur changes to mat.

So:

• matur = nominative form
• mat = accusative form

This is very common in Icelandic, especially with masculine nouns.

Yes, it is present tense on the surface: við kaupum ekki...

But Icelandic, like English, often uses the present tense for a current decision, plan, or near future situation.

So depending on context, this sentence can sound like:

• we aren't buying more food
• we won't buy more food
• we're not buying any more food

All of those are natural translations in English, even though the Icelandic verb form is present.

Because svo here is just linking two main clauses, and the second clause keeps normal word order:

• svo við kaupum ekki meiri mat

That is the ordinary subject + verb order.

Icelandic does often put the verb before the subject in some cases, but not every linking word causes that. With svo used this way, normal order is fine and expected.