What it means
なぜなら introduces an explanation for a statement that has just been made. It is usually followed by a reason clause ending in からだ or からです, meaning “because...” or “the reason is...”. The expression is more explanatory and structured than simply placing から after a clause, and it is common in writing, presentations, and careful speech.
Usage note
なぜなら normally refers back to a preceding statement, so it is often unnatural as the only clause in an exchange. The reason clause usually ends with からだ in familiar language or からです in polite language.
How to use it
私は早く帰りました。なぜなら、翌朝試験があるからです。
I went home early because there is an exam the next morning.
私は早く帰った。なぜなら、翌朝試験があるからだ。
I went home early because there is an exam the next morning.
Example sentences
私は毎朝図書館で勉強する。なぜなら、家では弟がゲームをしていて集中できないからだ。
I study at the library every morning. Because at home, my younger brother plays video games, so I cannot concentrate.
この薬は食後に飲んでください。なぜなら、空腹のまま飲むと胃に負担がかかるからです。
Please take this medicine after meals. The reason is that taking it on an empty stomach puts a strain on the stomach.
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