Góð orðabók er gagnleg þegar ég skrifa lengri texta.

It breaks down like this:

• Góð orðabók = a good dictionary
• er = is
• gagnleg = useful
• þegar = when
• ég skrifa = I write
• lengri texta = longer texts

So the structure is:

• subject: Góð orðabók
• verb: er
• predicate adjective: gagnleg
• time clause: þegar ég skrifa lengri texta

Because orðabók is a compound noun, and in Icelandic the gender of a compound usually comes from the last part.

• orð = word
• bók = book

Since bók is feminine, orðabók is feminine too.

This is a very useful rule in Icelandic: the final element of the compound usually determines the gender.

Because góð agrees with orðabók, which is:

• feminine
• singular
• nominative

Here Góð orðabók is the subject of the sentence, so the adjective has to be in the matching form.

Also, góð is the strong adjective form, which is used when the noun is indefinite here. In other words, orðabók means a dictionary, not the dictionary.

If the noun were definite, you would often get a weak form such as góða, for example in phrases like hin góða orðabók.

Gagnleg is an adjective describing the subject orðabók after the verb er.

After vera (to be), Icelandic uses a predicate adjective, and that adjective agrees with the subject.

So because orðabók is feminine singular, the adjective is also feminine singular:

• orðabók → feminine singular
• gagnleg → feminine singular

If the subject were neuter, you would expect gagnlegt instead.

It is an adjective.

That is because it describes orðabók: the dictionary is useful.

This is similar to English: we say The dictionary is useful, not The dictionary is usefully.

So after er, Icelandic uses the adjective form, not an adverb.

Because þegar introduces a subordinate clause.

In ordinary main clauses, Icelandic often follows a verb-second pattern. But in subordinate clauses, the order is usually more straightforward:

• þegar ég skrifa = when I write

So the subject ég comes before the verb skrifa.

That is why þegar ég skrifa sounds normal here.

Because for many Icelandic verbs, the 1st person singular present has the same form as the infinitive without að.

For this verb:

• infinitive: að skrifa = to write
• present, 1st person singular: ég skrifa = I write

So the form skrifa itself is not surprising. What tells you the function is the context:

• að skrifa = infinitive
• ég skrifa = finite verb, I write

Here þegar means when.

Because the sentence is a general statement, it can feel a bit like when or whenever in English:

• A good dictionary is useful when I write longer texts
• almost: ...whenever I write longer texts

So þegar is introducing the situation in which the dictionary is useful.

Texta is the accusative form of texti.

It is used because texta is the direct object of skrifa:

• ég skrifa texta = I write a text / texts

So the verb skrifa takes an object here, and that object appears in the accusative.

A useful extra point: texta can be either accusative singular or accusative plural in form, so context tells you which meaning is intended. In this sentence, the intended meaning is longer texts.

Lengri is the comparative form of langur:

• langur = long
• lengri = longer

So lengri texta means longer texts.

Notice that the stem changes:

• lang- → leng-

That is normal for this adjective and has to be learned as part of its comparison pattern.

Yes, but it would mean something different.

• lengri texta = longer texts
• langa texta = long texts

So:

• lengri is comparative
• langa is not comparative; it just means long in the appropriate agreeing form

If the meaning shown to you is longer texts, then lengri is the correct choice.

Because Icelandic does not have a separate indefinite article like English a/an.

So:

• orðabók can mean a dictionary
• góð orðabók can mean a good dictionary

If you want the dictionary, Icelandic usually adds the definite article to the end of the noun:

• orðabókin = the dictionary

So the lack of a separate word for a is completely normal.

Most naturally, it sounds like a general statement or habit.

Ég skrifa in the present tense often means:

• I write
• I am writing in some contexts
• I write / whenever I write as a habitual situation

So the sentence most naturally means something like:

• A good dictionary is useful when I write longer texts
• in the sense of a general truth or repeated situation

It does not have to mean only one specific writing session.