Hún spurði hvaða pils ég ætlaði að vera í á morgun.

Hvaða means which or what kind of, and it is used before a noun.

• hvaða pils = which skirt
• hvað on its own = what

So here you get hvaða because it is modifying pils.

A useful contrast:

• Hvað sagðirðu? = What did you say?
• Hvaða pils valdirðu? = Which skirt did you choose?

Because Icelandic very often expresses clothing with the idea of being in something.

• vera í pilsi = literally be in a skirt
• natural English translation: wear a skirt / be wearing a skirt

So að vera í is a very common way to talk about what someone has on.

Related pattern:

• fara í = put on / get into
• vera í = be wearing / be in

For example:

• Ég fer í jakkann. = I put on the jacket.
• Ég er í jakkanum. = I am wearing the jacket.

Because í belongs with vera in the expression vera í.

In this kind of question structure, Icelandic can leave the preposition at the end:

• hvaða pils ég ætlaði að vera í

That may feel unusual to an English speaker, but English actually does something similar:

• Which skirt was I going to be in?

You may also come across versions where the preposition stays in front, but the important thing is to recognize vera í as the unit.

Here að is the infinitive marker, basically the equivalent of English to.

• ætlaði að vera = intended to be / was going to be

So the structure is:

• ætla
  • að
    • infinitive

Examples:

• Ég ætla að fara. = I am going to go.
• Hún ætlaði að koma. = She was going to come.

This að is not the same as the preposition að meaning to/toward.

Ætlaði is the past tense of ætla.

In many contexts, ætla að means:

• intend to
• be going to

So ég ætlaði að vera í ... means something like:

• I intended to wear ...
• I was going to wear ...

In this sentence, was going to wear is usually the most natural English translation.

Because this is an embedded question after Hún spurði.

Direct question:

• Hvaða pils ætlaðir þú að vera í á morgun?
• Which skirt were you going to wear tomorrow?

Embedded question:

• Hún spurði hvaða pils ég ætlaði að vera í á morgun.
• She asked which skirt I was going to wear tomorrow.

In other words, once the question is inside a larger sentence, Icelandic uses subordinate-clause word order rather than main-question word order.

Because á morgun means tomorrow relative to the time of asking, not necessarily relative to the speaker's current moment.

So the timeline is:

• she asked in the past
• the wearing was planned for the next day after that

English does the same thing:

• She asked which skirt I was going to wear tomorrow.

So there is no contradiction between spurði and á morgun.

Because this is not a yes/no question. It is a which-question, and hvaða already introduces it.

So:

• Hún spurði hvaða pils ég ætlaði að vera í. = She asked which skirt I was going to wear.

But hvort is used for whether/if questions:

• Hún spurði hvort ég ætlaði að koma. = She asked whether I was going to come.

So:

• hvaða = for which/what type questions
• hvort = for whether/if type questions