なぜなら

なぜなら starts a sentence that gives the reason for the one before it: "the reason is", "because". It is なぜ + なら, "if you ask why", and the reason sentence closes with からだ or からです.

The closing からだ answers the なぜ, and the sentence sounds unfinished without it.

• × なぜなら、熱がある。 → なぜなら、熱があるからだ。

なぜなら or から

から in a single sentence puts the reason first. なぜなら splits it in two: the statement first, then the reason. It never comes before the statement it explains.

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なぜなら belongs to writing, presentations and arguments where you lay out reasons step by step. In conversation, なぜかというと is the neutral choice and だって the casual one. The reason can also close with ためだ or のだ, and なぜならば is a more formal spelling.