Kiu = who or which one?

Question

A friend of mine begins to learn Esperanton, and he asked me this question :

How to translate « Kiuj estas viaj instruistoj?»

I answered “Who are your teachers?”, but then, he asked me,

Why not « Which ones are your teachers? »?

I’ve no clue why. Can someone explain to me and my friend? Thanks!

Answer 1

Kiu means which one of some specific group.

• Kiu aŭto estas la via? : Which car is yours?

If no group is specified, it is assumed that we are talking about people in general.

• Kiuj estas estas viaj instruistoj? : Which people = Who are your teachers?

Your friend is asking about specific group.

• Kiuj el tiuj estas viaj instruistoj? : Which of those = Which ones are your teachers?

Answer 2

It could be both. When you ask kiuj you are singling them out, asking for the identity. The identity could be expressed for example as full names, or location, or otherwise.

Kiuj estas viaj instruistoj?

The answer could be:

John and Jane. (John kaj Jane)

or

The two standing by the exit door (la du kiuj staras ĉe la elirejo)