Ertu viss, eða ertu bara að giska?

A natural breakdown is:

• Ertu = are you
• viss = sure / certain
• eða = or
• ertu = are you
• bara = just / simply
• að giska = guessing (literally to guess, but here it works like English guessing)

So the structure is very close to English: Are you sure, or are you just guessing?

Ertu is the common combined form of ert þú.

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

In modern Icelandic, this kind of combination is very common in questions:

• Ertu... ? = Are you... ?
• Geturðu... ? = Can you... ?
• Viltu... ? = Do you want... ?

You may also see the separate form Ert þú..., but ertu is the normal everyday form.

Because this is a yes/no question.

In Icelandic, yes/no questions usually put the finite verb first:

• Þú ert viss. = You are sure.
• Ertu viss? = Are you sure?

So the word order changes in the same basic way it does in English.

Viss means sure, certain, or confident.

In this sentence, it is a predicate adjective after vera (to be), just like English be sure:

• vera viss = to be sure

So Ertu viss? is literally Are you sure?

A useful note: the form viss works for both masculine and feminine singular here, so the sentence is not marking the addressee’s gender.

Because the sentence is made of two coordinated clauses:

• Ertu viss
• eða ertu bara að giska

Each side has its own verb, so repeating ertu is completely natural and clear.

In English, we also often repeat the verb in this kind of contrast:

• Are you sure, or are you just guessing?

You may sometimes hear shorter, more elliptical phrasing in casual speech, but the full version with repeated ertu is very standard.

This is part of the very common Icelandic pattern vera að + infinitive.

Here:

• ertu = are you
• að giska = guessing / in the process of guessing

So ertu að giska? means are you guessing?

Even though giska is literally an infinitive (to guess), the whole pattern vera að + infinitive often corresponds to the English present continuous:

• Ég er að lesa. = I am reading.
• Hann er að vinna. = He is working.
• Ertu að giska? = Are you guessing?

So að is not random here; it is part of the construction.

Yes, very close.

The word bara means just, simply, or sometimes only. In this sentence, it adds the idea that the person may not really know the answer and is merely guessing.

So the tone is a bit skeptical:

• Ertu viss? = Are you sure?
• ... eða ertu bara að giska? = ... or are you just guessing?

You could, but it is not exactly the same.

• Ertu bara að giska? focuses on what the person is doing right now in this situation.
• Giskarðu bara? can sound more general, like Do you just guess? or Are you guessing?, depending on context.

If you want the same feeling as English Are you just guessing?, the original ertu bara að giska? is the most direct match.

Because the sentence joins two full clauses:

• Ertu viss
• eða ertu bara að giska

In Icelandic, a comma is commonly used before conjunctions like eða when they connect full clauses with their own finite verbs. So the comma helps show the division between the two parts of the question.

Yes. Giska is a normal everyday verb meaning to guess.

You can use it in simple sentences like:

• Ég er bara að giska. = I’m just guessing.
• Geturðu giskað? = Can you guess?
• Hún giskaði rétt. = She guessed correctly.

So this sentence uses very ordinary, useful Icelandic.