Ég borða brauðsneiðina með osti.

Because borða is the 1st person singular form of the verb að borða (to eat) in the present tense.

Present tense:

• ég borða = I eat
• þú borðar = you eat
• hann/hún/það borðar = he/she/it eats

So ég borða is the correct form for I eat.

brauðsneiðina can be broken down like this:

• brauð = bread
• sneið = slice
• brauðsneið = bread slice / slice of bread
• -ina = the definite article in this case ending

So:

• brauðsneið = a slice of bread
• brauðsneiðina = the slice of bread

Icelandic often builds words this way, especially compounds.

That -s- links the two parts of the compound:

• brauð
  • sneið → brauðsneið

This is very common in Icelandic compounds. It often reflects an old genitive relationship, similar to slice of bread in English. You do not translate the -s- directly; it is just part of how the compound is formed.

Because it is the direct object of the verb borða, so it is in the accusative case.

The sentence is:

• Ég borða brauðsneiðina = I eat the slice of bread

Since the slice of bread is what is being eaten, Icelandic puts it in the accusative.

Also, the noun is feminine singular definite, and in the accusative that gives brauðsneiðina.

Because Icelandic usually adds the definite article to the end of the noun.

So:

• brauðsneið = a slice of bread
• brauðsneiðin = the slice of bread
• brauðsneiðina = the slice of bread, in the accusative

Unlike English, Icelandic often does not use a separate word like the. It attaches the article to the noun instead.

Because the preposition með usually takes the dative case when it means with.

The noun is:

• ostur = cheese
• osti = cheese, in the dative singular

So:

• með osti = with cheese

This is one of the important things to memorize in Icelandic: many prepositions require a specific case.

Icelandic often leaves indefinite nouns untranslated in that sense.

So:

• með osti can mean with cheese
• English might naturally say with some cheese, but Icelandic does not need a separate word there

If it were definite, you would expect something like:

• með ostinum = with the cheese

So osti here is indefinite.

In practice, it most naturally means the bread slice has cheese with it or on it: I eat the slice of bread with cheese.

Grammatically, a phrase like með osti can sometimes feel a little flexible, just like in English. But in this sentence, the normal interpretation is that the bread slice is the thing that comes with cheese.

If you wanted to be extra clear in Icelandic, context usually does that.

This is the normal Icelandic subject–verb–object order:

• Ég = subject
• borða = verb
• brauðsneiðina = object
• með osti = prepositional phrase

So it follows a very familiar pattern for an English speaker:

• I eat the slice of bread with cheese

Icelandic can vary word order more than English in some contexts, but this sentence uses the straightforward neutral order.

Yes, but it would mean something slightly different.

• Ég borða brauð með osti = I eat bread with cheese
• Ég borða brauðsneiðina með osti = I eat the slice of bread with cheese

The original sentence is more specific:

• it is a slice
• and it is definite: the slice

So the difference is both in specificity and in what noun is used.

The dictionary forms are:

• brauðsneið = slice of bread
• ostur = cheese

In the sentence, they appear in changed forms because of case and definiteness:

• brauðsneiðina = accusative singular definite
• osti = dative singular indefinite

This is very normal in Icelandic, so it is useful to get used to identifying the base form behind the ending.

No. In this sentence, borða is a conjugated present-tense verb meaning eat with the subject ég.

The infinitive is:

• að borða = to eat

So:

• að borða = infinitive
• ég borða = I eat

They happen to look the same here except for the infinitive marker að.

It can do both, depending on context.

• Ég borða brauðsneiðina með osti can mean I am eating the slice of bread with cheese
• or I eat the slice of bread with cheese

Like many present-tense forms, Icelandic can express either a present action or a habitual/general one, depending on the situation. Context tells you which meaning is intended.