Hvert ertu að fara núna?

Because hvert asks about direction or destination: to where?

With a motion verb like fara (to go), Icelandic normally uses hvert rather than hvar.

• Hvert ertu að fara? = Where are you going?
• Hvar ertu? = Where are you?

So the difference is:

• hvar = where? (location)
• hvert = where to? (destination)

These three are a very useful set to learn together:

• hvar = where? / at where?
• hvert = where to?
• hvaðan = where from?

Examples:

• Hvar ertu? = Where are you?
• Hvert ertu að fara? = Where are you going?
• Hvaðan ertu að koma? = Where are you coming from?

English often uses just where, but Icelandic makes these distinctions more clearly.

Ertu is basically ert þú said and written together in a very common question form.

• ert = are (2nd person singular of vera, to be)
• þú = you

So:

• ert þú = are you
• ertu = a very common merged form of ert þú

Both are understandable, but ertu is extremely normal in everyday Icelandic.

Because Icelandic usually follows a verb-second pattern in main clauses.

In a question like this:

• Hvert takes the first position
• the finite verb ertu comes next
• then the subject is understood inside ertu / ert þú

So the structure is roughly:

• Hvert
  • ertu
    • að fara
      • núna

That is why Hvert þú ert að fara... sounds wrong.

Here að fara is part of the construction vera að + infinitive, which often expresses an action that is happening right now or is in progress.

So:

• ertu að fara = are you going / are you in the process of going

This is similar to the English progressive:

• are going
• are doing
• are eating

Important: the að here is not just a simple English-style to by itself. It is part of the pattern vera að + verb.

Yes, that is grammatical, and it can also mean Where are you going now?

But there is a difference in feel:

• Hvert ertu að fara núna? sounds very natural for something happening right now
• Hvert ferðu núna? can sound a bit simpler, sometimes slightly more general, or a little less focused on the ongoing action

In everyday speech, Hvert ertu að fara núna? is a very common way to ask this.

Núna means now or right now.

It adds a sense of immediacy:

• Hvert ertu að fara? = Where are you going?
• Hvert ertu að fara núna? = Where are you going now?

If the context already makes the time obvious, núna can be left out.

Because hvert already includes the idea of to where.

So Icelandic does not need an extra word meaning to here.

This is actually not so different from English, since English also usually says:

• Where are you going?

rather than:

• To where are you going?

Even though the directional idea is still there.

It is a normal, everyday sentence.

Using ertu makes it feel natural and conversational. A slightly fuller version would be:

• Hvert ert þú að fara núna?

That is also correct, but in ordinary speech and writing, ertu is extremely common and not strange at all.

In this kind of question, fara can sometimes feel like either:

• go
• head off
• leave

depending on context.

So Hvert ertu að fara núna? could be asked because someone is already on their way out, and in English you might naturally translate it as:

• Where are you going now?
• Where are you off to now?
• Where are you leaving for now?

The exact nuance depends on the situation.