Learn Turkish: Soyadınız nedir? - What is your surname?

The -ınız suffix is the formal or plural "your" (belonging to the pronoun siz). Since asking for a surname is usually an official or polite interaction, we use the formal possessive ending rather than the informal -ın.

Why use nedir instead of just asking ne?

Ne means "what". Adding the -dir suffix acts like the verb "to be" (is). While you could technically just end the sentence with ne, adding -dir makes the question sound more factual, formal, and complete, which perfectly matches the polite tone of soyadınız.

Could I say Soyadın ne? to mean the exact same thing?

Yes, but the tone changes completely! Soyadın ne? uses the informal "your" (-ın) and drops the formal -dir suffix. You would use that informal version with a child or someone your own age in a casual setting, whereas this sentence is what a receptionist, police officer, or bank teller would ask you.

What are the two root words that make up soyad?

It is a compound word made of soy (meaning ancestry, lineage, or family) and ad (meaning name). Put together, it literally translates to "family name".