Grammar Point
particle
prt 
familiar language
fam 
だい

is it?, isn't it?

marks wh-question (what, where, who)

Interrogative Word + Sentence + だい

だい is a sentence-final particle used in informal male speech to mark questions that contain an interrogative word (like who, what, where, when, why). It adds a softer or more familiar tone compared to using the question word alone, but it is specifically for questions asking for information, not for yes/no questions.

Verb / い-Adjective + んだい

When following a verb or an い-adjective, the explanatory particle 'の' (often contracted to 'ん') is placed before 'だい'.

What are you doing?

Where are you going?

Noun / な-Adjective + だい

When following a noun or a な-adjective, 'だい' can be attached directly to the word.

Who is that person?

When is the party?

Note

だい can only be used in questions that contain a question word (who, what, etc.). It cannot be used for yes/no questions. While primarily used by males in informal situations, it can sometimes sound slightly old-fashioned or characteristic of a specific regional dialect (like Tokyo).

Words: 
だい
Radicals of radicals