Translation guide
An emphatic phrase added to wh-questions to express surprise, disbelief, or strong curiosity. In Japanese, this is expressed with sentence-ending particles, adverbial phrases, or rhetorical constructions rather than a direct translation.
Express strong surprise or disbelief in a wh-question, like 'What in the world...?' or 'Why on earth...?'
The adverb いったい (一体) is the most common and neutral way to add emphasis to a wh-question. It can be used in both spoken and written Japanese, and is appropriate in most situations.
What in the world happened?
Express that something is impossible or absurd, like 'How in the world could I...?'
A rhetorical question pattern meaning 'How could I possibly...?' or 'As if I could...'. できるものか is casual; できようか is formal/literary.
There is no direct Japanese equivalent of 'in the world' as a phrase. Avoid literal translations like 世界で (in the world) in this context, as they will sound unnatural. Use the patterns above instead.
いったい is an adverb that explicitly adds emphasis, while んだ is a sentence-ending pattern that seeks an explanation and naturally conveys curiosity or surprise. They are often combined for strong emphasis: いったい何をしているんだ？
いったいなぜ彼は来なかったの？
Why in the world didn't he come?
Adding んだ (casual) or のだ (formal) to a wh-question seeks an explanation and naturally conveys emphasis or strong curiosity. Often used with いったい for even stronger effect.
What in the world are you doing?
Why in the world did you do that?
The sentence-ending particle の (casual, often with rising intonation) or のか (more masculine/forceful) adds a tone of surprise or demand for explanation. Common in spoken Japanese.
Where in the world are you going?
Who in the world said such a thing?
A more emphatic version of いったい, meaning 'what on earth' or 'what in the world'. Slightly more dramatic or exasperated.
What in the world is going on?
A rhetorical pattern that demands an explanation, often with frustration. というのだ is formal/written; っていうの is casual spoken.
Why in the world are you opposing it now?
How in the world could I do such a thing?
Literally 'there is no way that...', used to strongly deny possibility. Often corresponds to 'How in the world could...?' in English.
How in the world could he do such a thing? (There's no way he would.)