Japanese phrase
なぜ___を使ったのですか？ na ze ___ o tsuka tta no de su ka
Meaning
Why did you use ___?
When to use it
Asks for the reason someone used a particular thing. The polite wording is suitable when asking a teacher, coworker, or someone not well known.
Example dialogue
なぜこのアプリを使ったのですか？ na ze ko no a pu ri o tsuka tta no de su ka
Why did you use this app?
便利だからです。 ben ri da ka ra de su
Because it is convenient.