Why/Because なぜ、どうして、なぜなら

2 ways to say "why" are:

1. なぜ naze - why
2. どうして doushite - why

 

They are basically interchangeable and start at the beginning of the sentence and are followed by the question

なぜどうして) 私 の ケーキ を たべました か?
naze (doushite) watashi no ke-ki o tabemashita ka?
Why did you eat my cake?
[There isn't a "you" but obviously you wouldn't be asking yourself this question.]

なぜなら + reason or excuse + kara

なぜなら はら が へった から。
nazenara hara ga hetta kara.