"Почему" is used to ask about the reason or cause of an action or situation. It is similar to the English word "why" and is used to understand the cause or motive behind something that has already happened while "Зачем" is used to ask about the purpose or intention behind an action. It is similar to the English word "for what purpose" or "what for" and is used to understand the reason or goal behind an action that is about to happen or is planned.
In summary, "почему" is used to ask about the reason for something that has already occurred, while "зачем" is used to ask about the purpose or intention behind something that is going to happen or is being planned.

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