Accident or intentional? Interrogatives

One of the common mistakes seen in Japanese English learners is the confusion of the interrogatives "How...?" and "Why...?" Of course we know that HOW is asking in what way or manner, and WHY is asking the reason or the purpose, but the confusion pops up when we try to form questions because both words can be translated into Japanese as "どうして？　(DO-SHITE?)"

The mistake is often seen when a Japanese English learner tries to form a sentence in Japanese in his or her head and then tries to translate and say it in English. I once made this mistake so I will never forget it. I was talking to an English friend who told me he had broken his arm some months ago. I asked him,

"Why did you break your arm? "

My friend stared at me in amazement for a while and then said,

"Well, it was such a lovely day, a perfect day for breaking an arm so I thought I'd have a go.... WHY? WHAT are you thinking? I tripped over and broke it, it wasn't intentional!"

Only after his answer did I realize the mistake I had made. I should have asked HOW? and not WHY?, but I think my mind at the time was working in Japanese mode (my mind has two channels, Japanese and English mode).　I was translating

"どうして腕折ったの？"　(DO-SHITE break your arm?)

into English, and made the mistake.

A slip of an interrogative this time makes an accident  an intentional injury by a masochist.