Ég leita að samheiti í orðabókinni.

Because leita is commonly used as leita að + dative, meaning to look for / to search for something.

So:

• leita að samheiti = look for a synonym

Here að is a preposition, not the infinitive marker to.

It is in the dative, because the phrase leita að takes the dative.

A useful thing to know is that samheiti does not visibly change here, even though it is dative. Some Icelandic nouns have the same form in more than one case.

So even though the form looks simple, its grammatical role is still dative.

Because í can take either:

• dative for location: in
• accusative for motion into something: into

Here the meaning is in the dictionary, not into the dictionary, so Icelandic uses the dative:

• í orðabókinni = in the dictionary

If there were motion into something, accusative would be more likely.

It shows two things at once:

• the noun is definite: the dictionary
• the noun is in the dative singular

Breakdown:

• orðabók = dictionary
• orðabókin = the dictionary
• orðabókinni = in/to the dictionary

Icelandic usually adds the as an ending on the noun instead of using a separate word like English does.

Because Icelandic normally puts the definite article onto the noun itself.

So instead of a separate word like English the, Icelandic often uses endings such as:

• -inn
• -in
• -ið

and their case forms.

In this sentence, orðabókinni already contains the idea of the dictionary.

Because Icelandic has no indefinite article equivalent to English a/an.

So:

• samheiti can mean a synonym or just synonym, depending on context

That is completely normal in Icelandic.

• samheiti is neuter
• orðabók is feminine

Gender matters because it affects:

• article endings
• case endings
• adjective forms

For example, the feminine noun orðabók becomes orðabókinni in the dative definite singular, while a neuter noun would take different endings.

Because leita að takes the dative, you would need the dative definite form:

• að samheitinu = for the synonym

Compare:

• að samheiti = for a synonym
• að samheitinu = for the synonym

So the article changes with the case.

It can mean either, depending on context.

The simple present in Icelandic often covers both:

• I look for
• I am looking for

If you want to make the ongoing action especially clear, Icelandic often uses:

• Ég er að leita að samheiti í orðabókinni.

That more explicitly means I am looking for a synonym in the dictionary.

Yes. The sentence you have is the neutral order:

• Ég leita að samheiti í orðabókinni.

But Icelandic allows other orders for emphasis or style. For example:

• Í orðabókinni leita ég að samheiti.

That puts focus on in the dictionary.

A key rule in Icelandic main clauses is that the finite verb usually stays in second position.

Usually you should keep it.

Unlike some languages such as Spanish or Italian, Icelandic generally does not regularly drop subject pronouns in ordinary sentences.

So:

• Ég leita að samheiti... is the normal way to say it

Leaving out ég would usually sound incomplete unless the context is very special.

Here are the basic forms you would look up:

• ég = I
• leita = to look for / search for
• að = preposition used here with leita
• samheiti = synonym
• í = in
• orðabók = dictionary

This is especially useful because words in Icelandic often appear with endings that change for case and definiteness.