なぜ・どうして

 ABOUT: 

なぜ and どうして both mean "why" and are basically interchangeable. A third "why" is なんで which is used the same way as the other two but is more casual.

 Examples:

なぜ日本語(にほんご)を勉強(べんきょう)していますか？
Why are you learning Japanese?
[why | Japanese | studying?]

どうして、そんなに歴史(れきし)に詳(くわ)しいですか？
How come you know so much history?
[why | such an extent | history | detailed/well-acquainted with]