Interrogative Pronouns and Verbs

Interrogative Pronouns and Verbs
 

I've been searching everwhere for an answer to this but can't find it anywhere, maybe it's the wording I've been using in search engines or something. Anyway, I've been doing some exercises and right now I'm focussing on the interrogative pronouns. I'm translating 'who understands?' And 'who doesn't sing?' In the answers they translate to '¿quién comprende? And ¿quién no canta? My question is why do the verbs take the conjugation that would be used for he, she, you or it?

I think it's for the very same reason that your verbs in English are "doesn't" and "understands". The verbs of impersonal forms are conjugated in third person singular.

- Se dice que habrá una onda de calor.
It's being said that there will be a heatwave.

- ¿Cómo se pronuncia "shrimp"?
- How is "shrimp" pronounced? / How does one pronounce "shrimp"?

- Uno se siente muy bien aquí.
One feels very well here.

- ¿Quién tiene mi libro?
Who has my book.

- Está lloviendo.
It is raining.

- Alguien vino a buscarte.
Somebody came looking for you.

- Nadie me conoce en esta ciudad.
Nobody knows me in this city.

- He hablado con todos los jefes, pero ninguno me hace caso.
I've talked to all the bosses, but none of them listens to me.