Ég þarf að ákveða mig núna.

Because þarf is the 1st person singular present tense form of the verb þurfa (to need / have to).

So:

• ég þarf = I need
• þú þarft = you need
• við þurfum = we need

The dictionary form is þurfa, but after ég you use the conjugated form þarf.

Here að is the infinitive marker, like English to before a verb.

So:

• að ákveða = to decide

In Ég þarf að ákveða mig núna, the structure is:

• Ég þarf = I need
• að ákveða = to decide
• mig = myself
• núna = now

This is very common in Icelandic after verbs like þurfa:

• Ég þarf að fara. = I need to go.
• Við þurfum að bíða. = We need to wait.

Because ákveða sig is a common expression meaning to make up one’s mind / decide oneself.

So:

• ákveða eitthvað = to decide something
• ákveða sig = to decide / make up one’s mind

That means:

• Ég þarf að ákveða mig núna. = I need to make up my mind now.

If you say Ég þarf að ákveða núna, it sounds more like I need to decide now, but it may feel less complete unless the context already makes clear what kind of deciding is meant. The version with mig strongly suggests making up my mind.

Mig is the accusative form of ég (I / me).

The pronoun forms are:

• ég = nominative (I)
• mig = accusative (me)
• mér = dative (me)
• mín = genitive (my / of me in certain uses)

The verb ákveða takes a direct object in the accusative, so when the object is reflexive, you get:

• ákveða mig
• ákveða þig
• ákveða sig

So mig is there because it is the object of ákveða.

Yes, functionally it is reflexive in meaning, because it refers back to the subject ég.

In English we often say myself, but Icelandic usually uses the normal object pronouns for 1st and 2nd person:

• ég ... mig
• þú ... þig

For the 3rd person, Icelandic uses the special reflexive form sig:

• Hann þarf að ákveða sig. = He needs to make up his mind.
• Hún þarf að ákveða sig. = She needs to make up her mind.

So in this sentence, mig means something like myself.

Ákveða usually means to decide, to determine, or to settle something.

Examples:

• ákveða dagsetningu = decide a date
• ákveða verð = set/decide a price
• ákveða sig = make up one’s mind

So the core idea is decision, but the exact English translation depends on whether there is an object and what kind of object it is.

The basic word order is:

• Ég = subject
• þarf = finite verb
• að ákveða mig = infinitive phrase
• núna = adverb of time

So:

Subject + finite verb + infinitive phrase + time adverb

This is very normal Icelandic word order in a main clause.

Yes. Núna (now) is fairly flexible.

You can say:

• Ég þarf að ákveða mig núna.
• Núna þarf ég að ákveða mig.

Both are possible, but the emphasis changes.

• Ég þarf að ákveða mig núna. = neutral, with now added at the end
• Núna þarf ég að ákveða mig. = more emphasis on now

So the version you were given is a very natural default order.

Both can mean now, but núna often sounds a bit fuller or more conversational in everyday speech.

In many contexts they are interchangeable:

• Ég þarf að ákveða mig núna.
• Ég þarf að ákveða mig nú.

The version with núna is extremely common in spoken Icelandic. Nú can sometimes sound a little shorter, more pointed, or slightly more formal/literary depending on context.

A rough pronunciation guide is:

Ég þarf að ákveða mig núna
≈ yeg tharv ath ak-KVAY-tha mig NOO-na

A few helpful notes:

• Ég sounds roughly like yeg
• þ sounds like English th in thin
• ð often sounds like soft th in this, though in fast speech it may be weaker
• Stress usually falls on the first syllable of each word:
  • ÉG
  • ÞARF
  • Á-kveða
  • MIG
  • NÚ-na

The word ákveða is the trickiest one for many learners.

Yes. Icelandic þurfa often covers both ideas.

So Ég þarf að ákveða mig núna could be understood as:

• I need to make up my mind now
• I have to make up my mind now

Which English version is best depends on context. Icelandic does not always separate those two meanings as sharply as English sometimes does.

Here are a few useful comparisons:

• Ég þarf að ákveða mig núna. = I need to make up my mind now.
• Þú þarft að ákveða þig núna. = You need to make up your mind now.
• Hann þarf að ákveða sig núna. = He needs to make up his mind now.
• Hún þarf að ákveða sig núna. = She needs to make up her mind now.
• Við þurfum að ákveða okkur núna. = We need to make up our minds now.

This is a good sentence for seeing both verb conjugation and reflexive/object pronoun changes.