Ég á tvö pils, en ég man ekki hvaða pils er hreinna.

Here á is the 1st person singular present form of the verb eiga, which means to have / to own.

So:

• Ég á tvö pils = I have two skirts

The word á can also be a preposition meaning on, but here it is clearly a verb because it comes right after ég and is followed by a direct object, tvö pils.

Because pils is a neuter noun, and Icelandic numbers from 1–4 change form depending on gender.

For two, the main gender forms are:

• tveir for masculine
• tvær for feminine
• tvö for neuter

Since pils is neuter, you say tvö pils.

That is a very common source of confusion. Pils has the same form in the indefinite nominative/accusative singular and plural.

So:

• eitt pils = one skirt
• tvö pils = two skirts

In your sentence:

• tvö pils is clearly plural because of tvö
• hvaða pils er hreinna is singular because the verb is er (is), not eru (are)

So the second pils means which skirt, not which skirts.

Man is the present tense of muna, which means to remember.

So:

• ég man = I remember
• ég man ekki = I do not remember

This is just the normal present-tense form used with ég.

In a normal Icelandic main clause, ekki usually comes after the finite verb.

So:

• Ég man ekki = I do not remember

That word order is standard Icelandic. English uses do not remember, but Icelandic does not need a separate helping verb like do.

Hvaða means which here.

So:

• hvaða pils = which skirt

It introduces an indirect question inside the sentence:

• ég man ekki hvaða pils er hreinna
• I don’t remember which skirt is cleaner

A helpful thing to know is that hvaða does not change for gender or case, so the same form is used in many different situations.

Because hvaða pils is singular here: which skirt is cleaner.

So the verb must also be singular:

• er = is
• eru = are

Even though the speaker owns two skirts, the question inside the sentence is about choosing one skirt out of the two.

Hreinna is the comparative form of hreinn, meaning clean.

So:

• hreinn = clean
• hreinna = cleaner in this neuter singular context

It is hreinna because it agrees with pils, which is neuter singular here.

The structure is:

• hvaða pils er hreinna
• which skirt is cleaner

Icelandic uses the comparative here, just like English does, because two things are being compared.

You may come across hvor/hvort in Icelandic for the idea of which of two, but hvaða is the normal general word for which, and hvaða pils is perfectly natural here.

So for a learner, the safest takeaway is:

• hvaða = the common everyday word for which
• it works fine even when the context involves two things

Icelandic often repeats the noun where English might use one.

So English says:

• which skirt is cleaner?
• or which one is cleaner?

Icelandic can simply say:

• hvaða pils er hreinna

That is very natural. The repeated noun helps make the reference clear.