Ég undirstrika nafnorðið í setningunni.

Because for many Icelandic verbs ending in -a, the infinitive and the 1st person singular present tense look the same.

So:

• að undirstrika = to underline
• ég undirstrika = I underline

This is very common in Icelandic. You have to tell from the context whether the form is being used after að as an infinitive, or with a subject like ég as a finite verb.

Nafnorðið is the direct object of the verb undirstrika, so it is in the accusative case.

However, with this noun, the nominative and accusative singular definite forms are the same, so you see:

• nafnorðið = the noun

The reason it is accusative here is grammatical function: it is the thing being underlined.

That ending includes the suffixed definite article, which is how Icelandic usually says the.

The base noun is:

• nafnorð = noun

With the definite article:

• nafnorðið = the noun

So Icelandic usually does not use a separate word like English the. Instead, the article is attached to the end of the noun.

Because Icelandic normally puts the definite article onto the noun itself rather than using a separate word.

In this sentence:

• nafnorðið = the noun
• setningunni = the sentence

So the idea of the is already built into those word forms.

There is also a separate article form in Icelandic, but the attached article is much more common in ordinary sentences like this.

Because the preposition í can take different cases depending on meaning:

• accusative for motion into
• dative for location in

Here the meaning is location: the noun is underlined in the sentence, not moved into the sentence. So Icelandic uses the dative:

• í setningunni = in the sentence

If the meaning were movement into something, you would expect the accusative instead.

The base noun is:

• setning = sentence

It is a feminine noun.

In this sentence, it appears as setningunni because it is:

• singular
• definite
• dative
• after í with a location meaning

So setningunni means the sentence in the dative form required by the preposition.

Yes. This is a very normal Icelandic word order:

• Ég = subject
• undirstrika = verb
• nafnorðið = object
• í setningunni = prepositional phrase

So the structure is basically:

Subject + Verb + Object + Prepositional phrase

Icelandic word order can change for emphasis or style, but this version is straightforward and neutral.

For example, you could also say:

• Í setningunni undirstrika ég nafnorðið.

That is possible, but then Icelandic keeps the finite verb in second position.

Normally, yes, you should include ég.

Unlike some languages, Icelandic is not a language where subject pronouns are regularly omitted in ordinary sentences. Even though the verb form gives some information, speakers usually still say the pronoun:

• Ég undirstrika ... = normal

So for a standard full sentence, keeping ég is the best choice.

In Icelandic, accent marks are a normal part of spelling. They are not optional.

So:

• é
• í

are separate vowels, not just ordinary e and i with decoration.

These marks mainly show vowel quality, and sometimes help distinguish words. As a learner, the safest approach is to treat accented vowels as distinct letters that must be written correctly.

The letter ð is usually like the th in this, though in Icelandic it is often weaker than in English.

So in nafnorðið, the ð is not a d sound. It is a voiced dental sound, similar to English th in:

• this
• that
• other

A useful beginner rule is:

• þ is like th in thing
• ð is like th in this

It can do both.

Like English underline/underscore, undirstrika can be used:

• literally: to draw a line under a word
• figuratively: to emphasize or stress a point

In this sentence, the literal meaning fits naturally, but you may also see the verb used in more abstract contexts.