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¿cuando se utiliza el "what" y cuando el "which" en una pregunta?

Both refer to things. But "which" is used to ask for a choice from a limited set of possibilities.
e.g.
Which are you buying? (I can see two dresses. I don't know which dress you are buying)
What are you buying? (I can't see what it is.)

But many people say "what" meaning "which in informal English.
Sometimes these expressions are interchageable and it doesn't matter which of them you use.
But remember that which is for a limited choice.

Se usa "what" cuando quiere hacer preguntas sobre la identificación de una persona, objeto o acción-evento, tal como "What is your name?", "What is that?", "What are you doing?".

La palabra "which" se usa cuando tiene que escoger entre dos objetos/situaciones/personas etc., por ejemplo, "Which of these dresses do you like the best?", "Which one is the best one?".

Espero que le haya ayudado.

Hola TEZ ,

'what' significa " que " , "como"
Que haces = What are you doing
Que lio = What a mess
Como se llama? = What is it called

'which" significa "cual " ,"que"
Cual de vosotros? = Which one of you ?
Que quadro quieres? = Which picture do you want ?
la manzana ,que estaba sobre la messa = the apple, which is on the table
in cuyo caso = in which case