How do I say 'why'?

为什么 is grammatically straightforward — place it before the verb — but socially nuanced. A direct 你为什么 + action can land as accusatory, like demanding someone justify their behavior. Chinese has several ways to soften the question:

• 怎么 replaces 为什么 when you want to ask “how come” rather than “justify yourself.” 你怎么没告诉我？(“How come you didn’t tell me?”) sounds curious and slightly hurt; 你为什么没告诉我？sounds like an interrogation.
• 为啥 is the everyday colloquial version, common in speech and messages. It carries less formal weight than 为什么.
• Sometimes the most natural approach is to not ask “why” at all — Chinese often expresses confusion through a statement about not understanding (我不明白为什么…) rather than a direct why-question aimed at the other person.