Þú getur spurt hvern sem er hér.

Because after geta in Icelandic, you normally use the supine form of the main verb, not the infinitive.

So:

• spyrja = to ask
• spurt = the form used here after geta

That is why you get:

• Þú getur spurt ... = You can ask ...

This is very common:

• Ég get komið = I can come
• Við getum séð = We can see
• Hún getur hjálpað = She can help

So spurt here is completely normal.

Because spyrja takes a direct object, and that object is in the accusative case.

The base form is:

• hver = who / which one

But in the accusative masculine singular, it becomes:

• hvern

So in this sentence:

• spyrja hvern = ask whom

That is why the sentence has hvern sem er, not hver sem er.

As a whole, hvern sem er means anyone or whoever / whomever at all, depending on context.

It is a very common Icelandic pattern:

• hver sem er = anyone / whoever
• hvern sem er = anyone / whomever
• hvað sem er = anything
• hvar sem er = anywhere

So in your sentence, hvern sem er is best understood as one unit meaning anyone.

Not really in the normal word-for-word sense.

In this kind of expression, sem er is part of a fixed pattern that creates a free-choice meaning: anyone, anything, anywhere, and so on.

So although:

• sem often means that / who / which
• er is the verb is

in hver sem er you should mostly learn the whole expression together.

A useful way to think of it is:

• hver sem er = whoever it may be / anyone at all

And in your sentence that becomes hvern sem er because of the accusative.

Not with exactly the same meaning.

• einhvern usually means someone
• hvern sem er means anyone

So:

• Þú getur spurt einhvern hér. = You can ask someone here.
• Þú getur spurt hvern sem er hér. = You can ask anyone here.

The second one is broader and more like any person here, it doesn't matter who.

It can lean toward either ability/practical possibility or a softer kind of permission, depending on context.

• Þú getur spurt hvern sem er hér. = You can ask anyone here.

This can mean:

• you are able to ask anyone here
• it is fine/okay to ask anyone here

If you want to make the idea of permission clearer, you could say:

• Þú mátt spyrja hvern sem er hér. = You may ask anyone here / You're allowed to ask anyone here.

So:

• geta = can, be able to
• mega = may, be allowed to

English can often covers both ideas, but Icelandic can separate them more clearly.

Hér means here, and putting it at the end is very natural in this sentence.

• Þú getur spurt hvern sem er hér.

This is a normal, neutral word order.

You can move hér for emphasis, but that changes the feel a bit:

• Hér getur þú spurt hvern sem er. = Here, you can ask anyone.
• Þú getur hér spurt hvern sem er. = possible, but less natural in everyday speech

Yes, hérna is also possible:

• Þú getur spurt hvern sem er hérna.

Hérna is often a bit more conversational or colloquial than hér.

In a normal full sentence, you usually keep it.

So:

• Þú getur spurt hvern sem er hér.

is the standard form.

Icelandic does not usually drop subject pronouns the way some languages do. Even though getur already shows the person, þú is still normally expressed.

If you leave it out, the sentence may sound incomplete unless the context is very special.

The hver sem er part changes to match case, and sometimes gender, just like other Icelandic pronouns.

Common forms include:

• hver sem er = anyone (subject form)
• hvern sem er = anyone (accusative object, as in your sentence)
• hverjum sem er = to anyone / for anyone
• hvers sem er = of anyone

Examples:

• Hver sem er getur gert þetta. = Anyone can do this.
• Ég sá hvern sem er. = I saw anyone / just about anyone.
• Ég get talað við hverjum sem er. = I can talk to anyone.
  
  • More naturally: Ég get talað við hvern sem er, because við takes the accusative.

You may also see other related forms such as:

• hverja sem er = any woman / anyone feminine
• hvað sem er = anything
• hvar sem er = anywhere

So the core idea stays the same, but the form changes with grammar.