Learn Spanish (Latin America): ¿Te duele la espalda?

Because doler (to hurt) works just like gustar. Instead of saying 'You hurt', Spanish phrases it as 'Does the back cause pain to you?'. So, we use the pronoun te (to you) instead of the subject pronoun tú (you).

Why is it la espalda (the back) instead of tu espalda (your back)?

In Spanish, when talking about body parts, you almost always use the definite article (el, la, los, las) rather than a possessive word like tu (your). The te already makes it perfectly clear whose back is hurting!

If my friends' backs were hurting, would I change duele to duelen?

No, you would change te to les (to them). The verb duele matches the thing that is hurting (the back), not the person feeling the pain. Because it's still just one back per person, it stays duele. You only use duelen if multiple body parts are hurting, like las piernas (the legs).

Is duele an irregular verb?