Við eigum það til að gleyma afgangunum í ílátinu fram á næsta dag.

In this sentence, eiga það til að + infinitive is an idiomatic expression meaning to tend to, to be liable to, or to have a habit of doing something.

So:

• Við eigum það til að gleyma ... = We tend to forget ...

Here, eiga is not really about ownership in the usual sense. You should learn eiga það til að as a set phrase.

Examples:

• Hann á það til að sofa lengi. = He tends to sleep late.
• Ég á það til að vera sein/n. = I tend to be late.

No, það does not refer to a specific thing here. In eiga það til að, it is just part of the fixed idiom.

So you should think of:

• eiga það til að = to tend to

rather than trying to translate each word separately as own it to or something similar.

This is common in Icelandic: some expressions contain words that do not have a clear standalone meaning in that sentence.

Because after eiga það til, Icelandic uses the infinitive with að:

• eiga það til að gera eitthvað = to tend to do something

So:

• að gleyma = to forget

This is just the normal infinitive construction after this expression.

Afgangunum is the dative plural definite form of afgangar meaning leftovers.

Breakdown:

• afgangar = leftovers
• afgangana = the leftovers (accusative plural)
• afgangunum = the leftovers (dative plural)

It is in the dative here because the verb gleyma takes a dative object in Icelandic.

So Icelandic says, literally, something like:

• to forget the leftovers → að gleyma afgangunum

This is something learners need to memorize with the verb:

• gleyma einhverju = to forget something

Yes, gleyma normally takes a dative object.

Examples:

• Ég gleymdi lyklunum. = I forgot the keys.
• Hún gleymir nafninu. = She forgets the name.
• Við eigum það til að gleyma afgangunum. = We tend to forget the leftovers.

This is one of those Icelandic verbs where learning the case with the verb is very important.

Because í can take either:

• accusative for motion into
• dative for location in

Here the leftovers are already in the container, so this is location, not movement.

Therefore:

• í ílátinu = in the container

Compare:

• Við setjum afgangana í ílátið. = We put the leftovers into the container.
  
  • motion → accusative
• Afgangarnir eru í ílátinu. = The leftovers are in the container.
  
  • location → dative

It means something like:

• until the next day
• into the next day
• overnight, until the following day

In this sentence, it suggests the leftovers stay forgotten in the container and remain there until the next day comes.

Literally, fram á often has the sense of forward into/up to a later time.

Examples:

• fram á kvöld = until evening
• fram á nótt = into the night
• fram á næsta dag = until the next day

Because fram á here takes the accusative, and dagur changes form in the accusative singular.

So:

• nominative: næsti dagur = the next day
• accusative: næsta dag = the next day

After fram á in this time expression, you get the accusative:

• fram á næsta dag

The word order is quite natural. The sentence is structured like this:

• Við eigum það til = We tend to
• að gleyma afgangunum = to forget the leftovers
• í ílátinu = in the container
• fram á næsta dag = until the next day

So the full sentence means:

• We tend to forget the leftovers in the container until the next day.

The two prepositional phrases at the end give extra information:

• í ílátinu tells you where
• fram á næsta dag tells you for how long / until when

In practice, it works with the whole idea: the leftovers that are in the container are forgotten there.

English can be a little ambiguous with phrases like in the container, and Icelandic can be too, but here the most natural understanding is:

• we forget the leftovers that are sitting in the container

It is not saying that the act of forgetting happens physically inside the container, of course. It describes the leftovers’ location.

The dictionary form is ílát, which means container.

In the sentence:

• ílátinu = the container in the dative singular definite

Why that form?

• í with location takes dative
• the noun is definite: the container

So:

• í íláti = in a container
• í ílátinu = in the container

Yes, that is close in meaning, though it sounds more natural in English to say:

• We tend to...
• We have a tendency to...
• We’re liable to...

So Við eigum það til að gleyma afgangunum... is best understood as:

• We tend to forget the leftovers...

rather than a very literal translation.

A good learner takeaway would be these three patterns:

1. eiga það til að + infinitive
   = to tend to do something

2. gleyma + dative
   = to forget something

3. í + dative for location, í + accusative for motion

   • í ílátinu = in the container
   • í ílátið = into the container

And one helpful time expression:

4. fram á + accusative time expression
   = until / into a later time
   
   • fram á næsta dag = until the next day