Nadie vs. nada
The Spanish words nadie and nada are both negative indefinite pronouns, but they are not interchangeable. The key difference lies in what they refer to: one is used for people and the other for things.
Nadie
A1The word nadie means nobody, no one, or not anybody. It is used exclusively to refer to people and it does not change for gender or number.
Nadie vino a la reunión.
(Nobody came to the meeting.)
No vi a nadie en el parque.
(I did not see anyone in the park.)
Este secreto no se lo puedes contar a nadie.
(You cannot tell this secret to anyone.)
¿Quién sabe la respuesta? Nadie.
(Who knows the answer? No one.)
No dependo de nadie para ser feliz.
(I do not depend on anybody to be happy.)
Nada
A1The word nada means nothing or not anything. It is used exclusively to refer to objects, ideas, or actions. It can also be used as an adverb meaning not at all.
No hay nada en la nevera.
(There is nothing in the fridge.)
Nada es más importante que la salud.
(Nothing is more important than health.)
No entiendo nada de lo que dices.
(I do not understand anything you are saying.)
¿Qué te pasa? Nada, estoy bien.
(What's wrong with you? Nothing, I am fine.)
El hotel no era nada lujoso.
(The hotel was not at all luxurious.)
Summary
The difference is straightforward: nadie always refers to people (nobody), while nada always refers to things, ideas, or actions (nothing). If you can replace the word with no one, use nadie. If you can replace it with nothing, use nada.