Definition from Wiktionary, the free dictionary
- that is because; rhet. why you ask?
- Nazenara furui kara da.
- That is because it's old.
- なぜなら appears at the beginning of a sentence just to signal that a reason is given in the following sentence. It is always paired with some form of "because" at the end of the sentence such as からだ, から, ので etc., and なぜなら itself is not obligatory. Related to なぜかというと and somewhat less formal.