* なんて愚かだ (nante oroka da): This is a fairly direct translation, using the adjective 愚か (oroka - foolish). The なんて (nante) adds emphasis, similar to "how" in English. It expresses a strong feeling of disapproval.
* なんてばかだ (nante baka da): This is a more informal and slightly harsher way to say "how foolish," using ばか (baka - fool/stupid). Again, なんて (nante) emphasizes the foolishness.
* なんて愚かな (nante oroka na): Similar to the first option, but using the adjective in its na-form, making it a little less direct and possibly softer.
* どれほど愚かだったか (dore hodo oroka datta ka): This translates more literally to "how foolish it was," suitable for reflecting on a past foolish act.
* ずいぶん愚かだったね (zuibun oroka datta ne): This translates to "that was quite foolish," a more gentle and conversational way to express the idea.
The best choice will depend on the situation:
* Formal situations: Use なんて愚かな (nante oroka na) or どれほど愚かだったか (dore hodo oroka datta ka).
* Informal situations with friends: なんてばかだ (nante baka da) is acceptable.
* More polite and less judgmental: ずいぶん愚かだったね (zuibun oroka datta ne) is a good option.
Consider the relationship with the person you're speaking to and the level of formality required when choosing the best phrase.