Ég leita að andheitinu í orðabókinni.

For the meaning look for / search for, Icelandic usually uses the pattern leita að + dative. So it is helpful to learn leita að as a chunk.

In this sentence:

• leita = look/search
• að = the preposition that goes with this verb in this meaning

So the structure is not exactly copied from English. English says look for, while Icelandic says leita að.

No. It is the same word in form, but not the same function.

Here, að is a preposition meaning something like for / after in the expression leita að.

In að fara, að is an infinitive marker, closer to English to in to go.

So in this sentence, að is not introducing another verb. It belongs with leita.

Because andheiti is in the dative singular definite form.

That ending shows two things at once:

• dative case
• the definite article = the

The base noun is andheiti. In this sentence, leita að requires the next noun to be in the dative, so andheiti becomes andheitinu.

So:

• andheiti = antonym
• andheitinu = the antonym, in dative

Andheiti is neuter. Yes, that matters, because Icelandic endings depend on gender, number, and case.

The ending -inu fits a neuter singular definite noun in the dative. If the noun had a different gender, the ending would be different.

So when learning nouns in Icelandic, it is very important to learn them together with their gender.

Because it comes after í, and here í means in a place, not movement into a place.

With í, Icelandic usually works like this:

• dative for location: in
• accusative for motion toward / into

In this sentence, the speaker is searching in the dictionary, so it is a location. That is why Icelandic uses the dative: í orðabókinni.

Also, orðabókinni is the definite singular form of orðabók.

No. Í can take either dative or accusative, depending on the meaning.

A useful rule is:

• dative = being somewhere
• accusative = going into somewhere

So:

• í orðabókinni = in the dictionary → location → dative
• if it meant movement into something, accusative would be possible instead

This location-vs-motion difference is very common in Icelandic prepositions.

Yes. For this verb, the infinitive and the 1st person singular present happen to look the same.

So:

• að leita = to search
• ég leita = I search / I am searching

That can feel strange to English speakers, but it is normal in Icelandic. You tell the difference from the context and from the presence of the subject ég.

Because Icelandic often puts the definite article onto the end of the noun instead of using a separate word.

In this sentence:

• andheitinu = the antonym
• orðabókinni = the dictionary (in dative)

So the the is built into the endings -inu and -inni.

This is a very common feature of Icelandic nouns.

The sentence order can change, but the given order is a very normal, neutral one.

This sentence is:

• Ég = subject
• leita = verb
• að andheitinu = object/prepositional phrase
• í orðabókinni = location

Icelandic allows more movement than English, but in main clauses the finite verb usually stays in the second position. For example, you could also say:

Í orðabókinni leita ég að andheitinu.

That emphasizes in the dictionary more strongly. So the original sentence is not the only possible order, just the most straightforward one.