Orðaforðinn minn verður stærri þegar ég les mikið og athuga orð í orðabók.

In Icelandic, possessives often come after the noun, especially when the noun is definite.

• orðaforði = vocabulary
• orðaforðinn = the vocabulary
• orðaforðinn minn = my vocabulary

So the structure is literally something like the vocabulary mine, which is a normal Icelandic pattern. English usually puts the possessive first, but Icelandic very often puts it after the noun.

The ending -inn is the definite article, attached directly to the noun.

• orðaforði = vocabulary
• orðaforðinn = the vocabulary

Icelandic usually adds the onto the end of the noun instead of using a separate word like English does.

Here verður can be understood as becomes / gets.

• verða is a very common verb meaning to become, and in some contexts it can also help express the future.
• In this sentence, orðaforðinn minn verður stærri means my vocabulary gets bigger / becomes bigger.

So this is not just a simple static is sentence. It describes change over time.

stærri is the comparative form of stór.

• stór = big
• stærri = bigger

Because the sentence says the vocabulary is increasing, Icelandic uses the comparative: becomes bigger.

Here þegar means when.

So:

• þegar ég les mikið = when I read a lot

Be careful, because þegar can also mean already in other contexts. Icelandic learners often notice this double meaning, so you have to rely on context.

This is a normal subordinate clause:

• þegar = when
• ég = I
• les = read
• mikið = a lot

So the structure is straightforward: when I read a lot.

Unlike in some Germanic languages, Icelandic does not force the verb to the very end here. The order stays fairly natural: conjunction + subject + verb + adverb.

Yes. les is the 1st person singular present tense of lesa.

• lesa = to read
• ég les = I read

So þegar ég les mikið means when I read a lot.

mikið here functions adverbially and means a lot / much.

So:

• ég les mikið = I read a lot

This is a very common Icelandic pattern, where a neuter form like mikið is used in an adverb-like way to describe extent or degree.

Here athuga means something like check, look up, or examine.

In the context of language learning:

• athuga orð í orðabók = look up words in a dictionary

Depending on context, athuga can also mean check, inspect, or consider, so it is a flexible verb.

After the preposition í, Icelandic can use either:

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

Here there is no movement; it means in a dictionary, so orðabók is in the dative singular.

That is why you get:

• í orðabók = in a dictionary

orð is a neuter noun, and in this sentence it is the direct object of athuga.

The form orð is the same in nominative and accusative singular/plural in many uses, so it may look unchanged. That is very normal in Icelandic with some neuter nouns.

Here it simply means words.

Because the sentence means in a dictionary, not necessarily in the dictionary.

So:

• í orðabók = in a dictionary
• í orðabókinni = in the dictionary

If the speaker meant a specific dictionary, Icelandic would normally mark that directly on the noun with the definite ending.

Yes. That is exactly what is happening here.

• þegar ég les mikið og athuga orð í orðabók

The subject ég applies to both verbs:

• les
• athuga

So Icelandic, like English, often avoids repeating the subject when it is the same for both verbs.