Í nefnifalli er orðið oft einfaldast.

Because í takes the dative when it means location or state, and here it means something like in the nominative case.

• nefnifall = nominative case
• í nefnifalli = in the nominative

A useful rule is:

• í
  • dative = in/inside/within a place or state
• í
  • accusative = into, motion toward something

So this sentence uses nefnifalli because it is describing a grammatical state, not movement.

It is a fronted prepositional phrase, giving the setting for the statement: in the nominative.

Icelandic often moves this kind of phrase to the front for emphasis or to set the topic. When that happens, Icelandic still keeps the finite verb in the second position. That is why the sentence is:

• Í nefnifalli er orðið oft einfaldast.

and not something like:

• Í nefnifalli orðið er...

A more neutral order would be:

• Orðið er oft einfaldast í nefnifalli.

Both are possible, but the given sentence emphasizes in the nominative first.

Because Icelandic follows the verb-second rule in main clauses.

That means the finite verb usually comes in the second slot, no matter what comes first. Here the first slot is taken by Í nefnifalli, so the verb er must come next:

• 1st position: Í nefnifalli
• 2nd position: er
• then the subject: orðið

This is very common in Icelandic and is one of the biggest word-order differences from English.

Here orðið is the noun orð meaning word, with the suffixed definite article, so it means the word.

• orð = word
• orðið = the word

It can look identical to the past participle of verða (become), which is also orðið, but here the context makes it clear that it is the noun:

• it is the subject of er
• it matches the adjective einfaldast

So in this sentence, orðið definitely means the word.

Because Icelandic often uses the suffixed definite article to refer to a specific item or to something already understood from context.

The definite article in Icelandic is usually attached to the end of the noun:

• orð = a word / word
• orðið = the word

In grammar explanations, Icelandic may use a definite form where English might use either the word or even a more general phrasing like a word depending on context.

Because einfaldast is the form used here as a predicative adjective after er, agreeing with orðið.

• orðið is neuter singular
• so the adjective is also neuter singular: einfaldast

Compare:

• Orðið er einfaldast. = The word is simplest.
  
  • predicate use, after er
• einfaldasta orðið = the simplest word
  
  • attributive use, directly before the noun

So einfaldasta would be used when the adjective comes before the noun, not in this sentence structure.

Oft means often. It tells you that this is a frequent tendency, not an absolute rule.

So the sentence is not saying the word is always simplest in the nominative, only that this is often the case.

Its placement is also natural in Icelandic. In this sentence, it comes after the subject:

• Í nefnifalli er orðið oft einfaldast.

You could also see a different but still natural order, depending on emphasis, such as:

• Orðið er oft einfaldast í nefnifalli.

Yes. Icelandic adjectives normally agree with the noun they describe in gender, number, and often case.

Here:

• orðið is neuter singular
• einfaldast is also neuter singular

Since it comes after er, it is a predicate adjective, but it still agrees with the subject.

Yes. Er is the 3rd person singular present tense of vera (to be).

It is used here because the sentence is making a general statement:

• er = is

Icelandic, like English, often uses the present tense for general truths, patterns, and grammar explanations. So this is the normal tense you would expect here.