Ég undirstrika öll ný nafnorð í textanum áður en ég les hann aftur.

Because nafnorð is a neuter noun, and the words that go with it have to match its gender, number, and case.

In this sentence:

• nafnorð = noun / nouns
• it is plural
• it is the direct object of undirstrika, so it is in the accusative
• for neuter plural accusative, allur becomes öll
• and nýr becomes ný

So:

• öll = all, neuter plural
• ný = new, neuter plural
• nafnorð = nouns

That is why öll ný nafnorð is the correct form.

Because many Icelandic neuter nouns have the same form in the singular and plural, especially in the nominative and accusative.

So:

• nafnorð can mean a noun
• nafnorð can also mean nouns

You tell the difference from the surrounding words:

• nýtt nafnorð = a new noun
• öll ný nafnorð = all new nouns

Here, öll clearly shows that nafnorð is plural.

It is in the accusative, because it is the direct object of the verb undirstrika.

You are underlining what?

• öll ný nafnorð

That makes it the object, and verbs like undirstrika normally take the accusative.

One small extra point: with neuter plural, the nominative and accusative forms are often identical, so the form itself does not change, but the function in the sentence is still accusative.

Í textanum means in the text.

It is a prepositional phrase:

• í = in
• textanum = the text

Here it tells you where the nouns are being underlined.

A very important grammar point is that í can take different cases:

• dative for location: í textanum = in the text
• accusative for movement into something

Since this sentence describes location, not movement, Icelandic uses the dative.

Because textanum is the dative singular definite form of texti.

So the noun includes the meaning the text, not just text.

Breakdown:

• texti = text
• textanum = in the text / to the text, depending on context

In this sentence, because it follows í in a location sense, it means in the text.

Also notice that Icelandic usually adds the definite article to the end of the noun instead of using a separate word like English the.

Because texti is a masculine noun in Icelandic.

In Icelandic, pronouns agree with the grammatical gender of the noun they refer to, not with natural gender or with the English word it.

So:

• texti = masculine
• therefore the pronoun is hann when it refers back to texti

Even though English says it, Icelandic says hann because grammatically the word texti is masculine.

Because the pronoun’s case depends on its role in its own clause, not on the case of the noun it refers back to.

Earlier:

• textanum is in the dative because it follows í meaning location

Later:

• hann is the direct object of les
• direct objects are commonly in the accusative

So the same thing, the text, appears in different cases:

• í textanum = in the text, dative after a preposition
• les hann = read it, accusative object

This is very normal in Icelandic.

Áður en means before and introduces a subordinate clause.

So:

• áður en ég les hann aftur = before I read it again

It works a lot like English before followed by a full clause.

Notice that after áður en, Icelandic uses normal subordinate-clause word order:

• ég les hann aftur

The subject ég comes before the verb les.

Because les is a finite present-tense form, while lesa is the infinitive.

• að lesa = to read
• ég les = I read

In the sentence, the speaker is saying what they do, so Icelandic needs the conjugated form:

• ég les

not the infinitive:

• að lesa

This is the same difference as English I read versus to read.

Here undirstrika is the present tense, first person singular form of the verb að undirstrika.

So:

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

It is a regular-looking present-tense verb in this sentence:

• Ég undirstrika ... = I underline ...

A learner may notice that the infinitive and the ég form are the same here. That happens with many Icelandic verbs.

Because adverbs like aftur often come late in the clause in Icelandic, especially after the object.

So:

• ég les hann aftur = I read it again

This word order is natural and common.

Here aftur means again, not necessarily back in a physical sense.

Yes, but it is also very normal Icelandic word order.

The main clause is:

• Ég undirstrika öll ný nafnorð í textanum

Icelandic is a verb-second language in main clauses, which means the finite verb tends to come in the second position.

Here the order is:

1. Ég = subject
2. undirstrika = finite verb
3. the rest of the sentence

That is exactly what you would expect in a straightforward main clause.

A few sounds here often stand out to English speakers:

• ö sounds roughly like the vowel in French peur or German Hölle. There is no exact English equivalent.
• ð in áður is like the th in this
• ll in öll is not pronounced like normal English ll; in Icelandic it has a special sound, often something like a voiceless tl/dl-like sound depending on accent

Very rough approximations:

• Ég ≈ yehg
• öll ≈ utl / otl-like
• áður ≈ OW-thur
• hann ≈ hahtn with a short a
• aftur ≈ ahf-tur

Those are only approximations, but they can help at first.