Við erum tíu alls.

• Við = we
• erum = are (the we form of the verb að vera, to be)
• tíu = ten
• alls = in total / altogether

So the whole sentence means something like We are ten altogether or more natural English There are ten of us altogether.

Because Icelandic verbs change depending on the subject.

The verb að vera (to be) is irregular, and erum is the form used with við (we).

Present tense:

• ég er = I am
• þú ert = you are
• hann/hún/það er = he/she/it is
• við erum = we are
• þið eruð = you are
• þeir/þær/þau eru = they are

So Við erum is simply the correct agreement: we are.

Yes. við can be:

• a pronoun meaning we
• a preposition meaning with

In Við erum tíu alls, it is clearly the pronoun we, because it is the subject of the verb erum.

So:

• Við erum... = We are...
• með okkur = with us
  (not við okkur in this meaning)

This is a common thing to watch out for, because the same spelling can belong to different parts of speech.

It does not need one. Icelandic, like English, can leave the noun understood when the context makes it obvious.

So Við erum tíu alls literally looks like We are ten altogether, but the natural sense is There are ten of us altogether.

You could add a noun in some contexts, for example:

• Við erum tíu manns.

But in everyday speech, just Við erum tíu is often enough when everyone already knows you are talking about the number of people.

Alls here means in total, altogether, or all told.

It adds the idea that ten is the final total number, not just one part of a larger group.

So:

• Við erum tíu. = We are ten.
• Við erum tíu alls. = We are ten altogether / ten in total.

It is a small word, but it gives a sense of summing everything up.

Yes, that is also possible.

Both:

• Við erum tíu alls
• Við erum alls tíu

can mean We are ten altogether.

The placement of alls can vary a bit, and the difference is mostly about style or emphasis rather than basic meaning. Learners will probably hear both kinds of placement with adverbs in Icelandic.

That said, if you are learning the sentence as given, Við erum tíu alls is perfectly fine and natural.

Some Icelandic numbers do change form depending on gender and case, but tíu is much simpler than numbers like einn, tveir, þrír, and fjórir.

In this sentence, tíu stays tíu.

For a beginner, the important point is:

• tíu = ten
• in a sentence like this, you can use it directly after the verb

So you do not need to worry about changing tíu here.

Because English usually prefers there are ten of us when talking about how many people are in a group.

Icelandic can say this more directly with we are ten:

• Við erum tíu.

A very literal translation is helpful for structure:

• Við = we
• erum = are
• tíu = ten

But the most natural English translation is often:

• There are ten of us altogether.

So the Icelandic structure and the most natural English structure are not exactly the same, even though they express the same idea.

A rough guide for an English speaker:

• Við ≈ vith
  The ð is like the th in this, though in actual speech it can be softer or less fully pronounced.
• erum ≈ EH-rum
• tíu ≈ TEE-yuh
• alls ≈ alts or ahls depending on how carefully you are listening and to whom

A rough full pronunciation:

• Vith EH-rum TEE-yuh als

This is only an approximation. Icelandic pronunciation has details that English spelling cannot show well, but this should help you recognize the sentence when you hear it.

Yes. It is a very normal kind of sentence.

Icelandic often uses vera (to be) with a number to say how many people are present or included:

• Við erum þrjú. = There are three of us.
• Við erum fimm. = There are five of us.
• Við erum tíu alls. = There are ten of us altogether.

So this is a useful pattern to remember:

• Við erum + number
• optionally add alls for in total / altogether