Formal:
* 为什么 (wèishénme) - This is the most common and versatile way to say "why". It's suitable for most situations, especially formal ones.
* 为何 (wèihé) - This is a more literary and formal way to say "why", often used in writing or speeches.
Informal:
* 怎么 (zěnme) - This is a more casual way to say "why", often used in spoken language. It can also mean "how" depending on the context.
* 干吗 (gànma) - This is a very informal way to say "why", often used among friends or family.
Other:
* 因为 (yīnwèi) - This means "because" and can be used to explain the reason for something.
Examples:
* 你为什么迟到了？(nǐ wèishénme chídào le?) - Why are you late?
* 为何要这样做？(wèihé yào zhèyàng zuò?) - Why do you want to do this?
* 你怎么不告诉我？(zěnme nǐ bù gàosu wǒ?) - Why didn't you tell me?
* 干吗要生气？(gànma yào shēngqì?) - Why are you getting angry?
* 因为我很忙。(yīnwèi wǒ hěn máng.) - Because I'm very busy.
The best way to choose the right word is to consider the context and the formality of the situation.