Ég ruglast stundum á því hvaða forsetningu ég á að nota.

Að ruglast á einhverju means to get confused about something, to mix something up, or to be unsure which is which.

In this sentence:

• Ég ruglast = I get confused
• á því ... = about/over that ...

So Ég ruglast stundum á því ... means that the speaker sometimes gets confused about the thing mentioned in the clause that follows.

A useful comparison:

• að rugla = to confuse/mix up something
• að ruglast = to get confused / make a mistake

So Icelandic uses ruglast because the speaker is the one becoming confused.

This is a very common Icelandic structure.

The verb phrase is ruglast á + something. When that something is not just a noun, but a whole clause, Icelandic often inserts því after the preposition:

• á því hvaða forsetningu ég á að nota

Here, því works like a placeholder after the preposition á, introducing the clause that follows.

So the structure is roughly:

• ruglast á því
  • clause
• get confused about
  • clause

English usually does not need a word like því here, but Icelandic often does.

Hvaða means which or what kind of, depending on context. Here it means:

• hvaða forsetningu = which preposition

It is used before a noun when you are asking someone to identify one out of a set.

Examples:

• Hvaða bók viltu? = Which book do you want?
• Hvaða forsetningu á ég að nota? = Which preposition should I use?

In your sentence, it introduces an embedded question:

• hvaða forsetningu ég á að nota
  = which preposition I should use

Forsetningu is the accusative singular form of forsetning.

That happens because it is the direct object of nota:

• að nota eitthvað = to use something

So:

• forsetning = nominative
• forsetningu = accusative

Even though there is an á earlier in the sentence, forsetningu is not the object of á. It belongs to the clause ég á að nota and is the thing being used.

So the grammar is:

• á því = tied to ruglast á
• hvaða forsetningu ég á að nota = embedded question
• forsetningu = object of nota

Yes, eiga often means to own, but in the structure eiga að + infinitive, it means:

• to be supposed to
• should
• to be meant to

So:

• ég á að nota = I should use / I am supposed to use

This is a very common Icelandic construction.

Examples:

• Ég á að fara núna. = I should leave now.
• Þú átt að læra. = You are supposed to study.

So in your sentence, the speaker is not saying I own to use anything. This is the modal-like meaning of eiga að.

Because this is an embedded question, not a direct question.

Compare:

• Direct question:
  Hvaða forsetningu á ég að nota?
  = Which preposition should I use?

• Embedded question:
  Ég ruglast stundum á því hvaða forsetningu ég á að nota.
  = I sometimes get confused about which preposition I should use.

In embedded questions, Icelandic does not use the same question word order as in direct questions. The subject usually comes before the finite verb:

• ég á not
• á ég

This is very similar to English:

• direct: Which preposition should I use?
• embedded: ... which preposition I should use

Stundum means sometimes.

In this sentence:

• Ég ruglast stundum ... = I sometimes get confused ...

Its position here is very natural and neutral. But Icelandic adverbs can often move around somewhat, depending on emphasis and style.

Possible alternatives include:

• Stundum ruglast ég á því ...
• Ég ruglast á því stundum ...

All of these can work, but Ég ruglast stundum ... is a very normal everyday order.

The -st ending is a very important Icelandic feature. It often creates what is called a middle voice form.

In practice, -st verbs can have meanings like:

• something happens by itself
• someone undergoes the action
• a reflexive-like meaning
• a passive-like meaning

Here:

• rugla = to confuse / to mix up
• ruglast = to get confused / to be mistaken

So ruglast is not exactly the same as English reflexive confuse oneself, but that is close enough to help you understand it.

A useful contrast:

• Ég rugla nöfnum. = I mix up names.
• Ég ruglast á nöfnum. = I get confused about names / I mix names up.

Not naturally with the same meaning.

If you want to say I get confused, ruglast is the normal choice.

• Ég ruglast stundum á því ... = I sometimes get confused about ...

Using rugla instead would usually suggest that you are confusing something, not that you become confused.

So for this sentence, ruglast is the right verb form.

No, not reliably. This is one of those cases where Icelandic uses a preposition that simply has to be learned with the verb.

In English, you might say:

• confused about
• confused over
• unsure about

But in Icelandic, the idiomatic pattern is:

• ruglast á einhverju

So it is best to learn it as a chunk:

• að ruglast á einhverju

This is especially important with verbs and prepositions, because Icelandic often does not match English exactly.

Yes, very much so.

The sentence sounds natural and idiomatic for someone talking about learning Icelandic or any language. It is especially useful when talking about prepositions, case patterns, or grammar choices.

Very similar sentences could be:

• Ég ruglast stundum á kyninu.
  = I sometimes get confused about grammatical gender.

• Ég ruglast stundum á fallinu.
  = I sometimes get confused about the case.

• Ég ruglast stundum á því hvaða orð ég á að nota.
  = I sometimes get confused about which word I should use.

So this sentence is a very practical model to remember.