How; in what way or by what means (used to ask about method or manner).
明日はどうして会社へ行く？電車、それとも車？
How will you get to work tomorrow? By train or by car?
この機械はどうして動くのか、説明書を見てもよく分からない。
I don't really understand how this machine works even after looking at the manual.
料理はどうして作るの？簡単なレシピを教えてください。
How do you make the dish? Please tell me a simple recipe.
Why; for what reason or purpose (used to ask about cause or motive).
どうして昨日来られなかったの？何かあったの？
Why couldn't you come yesterday? Did something happen?
どうしてそんなことを言ったのか、理由を聞かせてください。
Please tell me why you said something like that.
どうして彼が遅刻したのか、電話で確認してみる。
I'll call to check why he was late.
(Used with negation) strongly indicates that something cannot possibly happen or be done; no matter what, it is impossible.
あのプロジェクトは時間が足りなくて、どうしても間に合わない。
We don't have enough time for that project; we simply cannot finish it in time.
高すぎて、どうしても買えない。
It's too expensive; I absolutely can't buy it.
An interjection expressing surprise, disbelief, or puzzlement (often repeated as 'どうして、どうして').
どうして！そんなことが本当にあったの？
What! Did that really happen?
どうして、どうして彼がそんなことをするのか分からない。
I just don't understand why he would do such a thing.