Hún skrifar nýjan orðaforða í glósubókina sína og skrifar líka hvort orðið er í eintölu eða fleirtölu.

Because Icelandic adjectives must agree with the noun they describe in gender, number, and case.

Here, orðaforða is:

• masculine
• singular
• accusative (because it is the direct object of skrifar)

So nýr changes to nýjan.

Orðaforði is the nominative form, the form you usually see in a dictionary.

In this sentence, it is the direct object of skrifar, so it must be in the accusative:

• nominative: orðaforði
• accusative: orðaforða

This is a common pattern for masculine nouns of this type.

Skrifar is the present tense, third-person singular, of skrifa.

Like many Icelandic present-tense forms, it can mean either:

• writes (habitually / generally)
• is writing (right now)

The wider context tells you which meaning is intended.

This is about the case used after í.

With í:

• accusative often suggests movement or direction into
• dative often suggests location in

So:

• í glósubókina = into the notebook
• í glósubókinni = in the notebook

Here the idea is that she is writing something into the notebook, so the accusative form glósubókina is used.

Because Icelandic uses the reflexive possessive sinn when the possessor is the same as the subject of the clause.

Here:

• subject: Hún
• the notebook belongs to that same person

So Icelandic uses sína = her own

If you said hennar, it would usually mean her in the sense of some other woman's, not the subject's own.

Also, sína agrees with glósubókina, not with hún. Since glósubókina is feminine singular accusative, the correct form is sína.

Because Icelandic often keeps the verb in both parts of a coordinated sentence when each part has its own complement.

Here the sentence has two linked actions:

• skrifar nýjan orðaforða í glósubókina sína
• skrifar líka hvort orðið er í eintölu eða fleirtölu

Repeating skrifar makes the structure clear and natural. English can do the same: She writes ... and also writes ...

Líka means also or too.

In this sentence, it means that she does one more thing in addition to writing down vocabulary.

The placement skrifar líka is very natural in Icelandic. Adverbs like líka often come after the finite verb.

So:

• skrifar líka = also writes

Here hvort means whether.

It introduces an indirect question:

• hvort orðið er í eintölu eða fleirtölu
• whether the word is in the singular or plural

This is not the same as English which in ordinary use. In this sentence, hvort is the standard word for introducing a whether-clause.

Because after hvort, you have an embedded clause, not a main question.

In an Icelandic main question, you often get inversion:

• Er orðið í eintölu? = Is the word singular?

But in an indirect question introduced by hvort, the word order is normal subordinate-clause order:

• hvort orðið er í eintölu = whether the word is singular

So orðið er is exactly what you should expect here.

Because orðið is the definite form: the word.

The noun is:

• indefinite: orð
• definite: orðið

Here it is the subject of er, so it is in the nominative singular, and the sentence is talking about a specific word being checked for number. That is why the definite form orðið is natural.

Because eintala and fleirtala are being used as grammar categories, in a general sense.

So:

• í eintölu = in the singular
• í fleirtölu = in the plural

Icelandic often leaves out the article in expressions like this when naming categories or forms.

Normally, no. Icelandic usually requires the subject to be stated.

So:

• Hún skrifar = She writes

You would not normally drop hún in a regular declarative sentence. Icelandic is not a language where subject pronouns are commonly omitted just because the verb ending shows the person.

Yes, very much so.

In this sentence, nýjan orðaforða means new vocabulary. Even though English learners may think of vocabulary as a collection of separate words, Icelandic can also use orðaforði as a single noun referring to vocabulary as a whole.

So the sentence is not necessarily about one single new word only; it is about writing down new vocabulary items.