Þetta er ekki gott umræðuefni í hádeginu, því allir eru orðnir of svangir til að ræða það rólega.

Þetta means this.

In this sentence, it points to the whole situation or topic being talked about, not necessarily to a single clearly named object right before it. Icelandic often uses þetta er ... just like English uses this is ....

It is in the neuter singular form, which is very common when Icelandic refers to:

• a general situation
• an idea
• something unspecified
• an entire statement

So Þetta er ekki gott umræðuefni ... = This is not a good topic of discussion ...

Because gott has to agree with umræðuefni, and umræðuefni is a neuter singular noun.

The adjective góður changes form depending on gender, number, and case:

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

So:

• góður maður = a good man
• góð bók = a good book
• gott umræðuefni = a good discussion topic

Here, umræðuefni is neuter, so gott is the correct form.

Umræðuefni means topic of discussion, discussion topic, or simply subject to talk about.

It is a compound noun:

• umræða = discussion
• efni = material, subject matter, topic

So the word literally suggests something like discussion-subject.

This kind of compound noun is extremely common in Icelandic.

Also, efni is a neuter noun, and in compounds like this the whole compound is also neuter, which is why you get gott umræðuefni.

Í hádeginu means at lunch, at lunchtime, or at noon, depending on context.

Here are the parts:

• hádegi = noon / midday
• í hádeginu = at noon / at lunchtime

The ending -nu is the definite article attached to the noun, so hádeginu means the noon / the lunchtime in form, but idiomatically it often just means at lunchtime.

Also, after í in a stationary time/location sense, Icelandic often uses the dative, which is why you get hádeginu here.

So:

• í hádegi can exist in some contexts
• í hádeginu is a very natural idiomatic way to say at lunchtime / at noon

Here því means because.

That can be confusing, because því can also be:

• the dative form of það = it / that
• an adverb/conjunction in other expressions

In this sentence, it is a conjunction introducing a reason:

• Þetta er ekki gott umræðuefni í hádeginu, því ...
• This is not a good discussion topic at lunch, because ...

So although the form is identical, the function is different. You understand which one it is from the sentence structure.

Orðnir is the past participle of verða, which usually means to become.

The combination vera + orðinn often expresses a resulting state:

• ég er orðinn þreyttur = I have become tired / I am now tired
• þau eru orðin svöng = they have become hungry / they are now hungry

So:

• allir eru orðnir of svangir = everyone has become too hungry

This is more expressive than simply saying allir eru of svangir. It suggests a change:

• they were not so hungry before
• now they have reached the point of being too hungry

Because they agree with allir.

Allir means everyone / all people, and grammatically it is plural. In Icelandic, when referring to a mixed group or people in general, the masculine plural is often used by default.

So:

• allir = masculine plural
• orðnir = masculine plural
• svangir = masculine plural

These words all match each other.

Compare:

• allir eru orðnir svangir = everyone has become hungry
• allar eru orðnar svangar = all of them (feminine group) have become hungry
• öll eru orðin svöng = all of them (neuter group / mixed things, etc.) have become hungry

This is an important difference between English and Icelandic.

In English, everyone is grammatically singular:

• everyone is hungry

In Icelandic, the equivalent idea is often expressed with allir, which is grammatically plural:

• allir eru svangir = everyone is hungry

So even though the meaning is similar to English singular everyone, the Icelandic grammar is plural.

That is why you get:

• allir eru not
• allir er

This is the pattern:

of + adjective + til að + infinitive

It means:

too + adjective + to + verb

So:

• of svangir = too hungry
• til að ræða það = to discuss it
• rólega = calmly

Together:

• of svangir til að ræða það rólega
• too hungry to discuss it calmly

This is a very useful Icelandic structure. For example:

• Hann er of þreyttur til að vinna. = He is too tired to work.
• Ég er of upptekinn til að koma. = I am too busy to come.

Because til að is a fixed grammatical combination meaning in order to / to in this kind of construction.

Here:

• til að ræða = to discuss

In Icelandic, some infinitives appear:

• with að
• without að

It depends on the verb or structure.

For example:

• Ég ætla að fara. = I intend to go.
• Ég get farið. = I can go.
  No að after geta

In this sentence, til að is required by the expression:

• of svangir til að ræða það rólega

So you should learn til að + infinitive as a unit.

Það means it, and it refers back to umræðuefni — the discussion topic.

So:

• ræða það = discuss it

Even though umræðuefni is a neuter noun, that fits perfectly, because það is the neuter pronoun form.

This kind of pronoun reference works much like English:

• the topic ... discuss it

Because rólega is an adverb, not an adjective.

Here it describes how they would discuss the topic:

• ræða það rólega = discuss it calmly

The adjective is:

• rólegur = calm

The adverb is:

• rólega = calmly

Compare:

• Þeir eru rólegir. = They are calm.
  Here you need an adjective, agreeing with þeir.
• Þeir tala rólega. = They speak calmly.
  Here you need an adverb, because it describes the action.

In your sentence, the word modifies ræða (to discuss), so the adverb rólega is correct.

Because því here is a subordinating conjunction meaning because, and the clause after it has normal subordinate-clause word order.

So:

• því allir eru orðnir of svangir ...
• because everyone has become too hungry ...

This is different from the normal main-clause verb-second pattern that Icelandic often uses.

Very roughly:

• Main clause: something often comes first, then the finite verb
• Subordinate clause after words like því, að, ef, etc.: the structure is more like a regular subject + verb order

So the sentence is behaving normally here.

It can suggest either, depending on context, but in this sentence at lunch / at lunchtime is probably the best interpretation.

Why? Because the sentence talks about:

• people becoming very hungry
• not being able to discuss something calmly

That makes the mealtime context especially natural.

So while hádegi literally relates to noon / midday, í hádeginu often works idiomatically as at lunchtime.