Hur uttalas ditt efternamn?

Because efternamn is an ett-word in Swedish: ett efternamn.

Swedish possessives change to match the grammatical gender of the noun:

• en-word: din
• ett-word: ditt

So:

• din bok = your book
• ditt efternamn = your last name

uttalas means is pronounced.

It comes from the verb uttala, which means to pronounce.
The -s ending makes it passive, so:

• uttala = to pronounce
• uttalas = is pronounced

So the sentence is structured very much like English How is your last name pronounced?

Because Swedish often prefers this passive construction in this kind of question.

So instead of asking literally:

• How do you pronounce your last name?

Swedish commonly says:

• How is your last name pronounced?

That is why Hur uttalas ditt efternamn? sounds natural and idiomatic.

In Swedish questions with a question word, the usual order is:

• question word + verb + subject

So here:

• Hur = how
• uttalas = is pronounced
• ditt efternamn = your last name

This is very similar to English:

• How is your last name pronounced?

So the structure is not strange once you think of uttalas as is pronounced.

Yes, efternamn is one word.

Swedish forms compound nouns very often, and they are usually written as a single word. Here:

• efter = after
• namn = name

Together:

• efternamn = surname / last name

So unlike English, where last name is two words, Swedish uses one compound word here.

A rough learner-friendly pronunciation is:

hur ut-TA-las dit EF-ter-namn

A few notes:

• hur has a Swedish u sound that does not exist in normal English; it is rounded, somewhat like German ü
• uttalas has the main stress on TA
• ditt sounds close to English dit
• efternamn has the main stress on the first syllable: EF

If you want to sound more natural, focus especially on:

• short i in ditt
• clear stress in utTA-las
• first-syllable stress in EF-ter-namn

Very often, yes.

In this sentence, hur means how in the sense of in what way:

• Hur uttalas...? = How is ... pronounced?

You will also see hur in many other common questions:

• Hur mår du? = How are you?
• Hur gammal är du? = How old are you?
• Hur gör man? = How do you do it?

So hur is a very useful question word.

It is neutral and completely normal.

Modern Swedish usually uses du-style forms with almost everyone, so ditt is standard even in polite situations. You do not usually need a special formal version for one person.

So Hur uttalas ditt efternamn? is a natural, polite question in everyday Swedish.

Yes, you may hear that too, and it can sound natural in conversation.

But there is a slight difference in feel:

• Hur uttalas ditt efternamn? = How is your last name pronounced?
• Hur säger man ditt efternamn? = How do you say your last name?

The first one is a bit more exact and directly about pronunciation.
The second one is more conversational.

Both can refer to a surname or family name.

• efternamn = last name / surname
• familjenamn = family name

In everyday Swedish, efternamn is very common and completely natural in this sentence.

Yes. In English, efternamn can be translated as either:

• last name
• surname
• sometimes family name

So Hur uttalas ditt efternamn? can be understood as:

• How is your last name pronounced?
• How is your surname pronounced?

They mean the same thing here.

Yes. If you are talking to several people, you would normally use ert instead of ditt.

So:

• Hur uttalas ditt efternamn? = your last name? (one person)
• Hur uttalas ert efternamn? = your last name? (several people)

That is because ert is the possessive form for ni.