だい (informal WH-question)

• Reading: だい
• Romaji: dai
• Formation: WH-word + ～ + だい

Meaning

A sentence-final particle used in informal male speech to mark questions containing an interrogative word (what, where, who, etc.). It adds a casual, familiar tone to information questions.

だい is the information-question counterpart to かい. While かい marks yes-no questions, だい marks questions that contain an interrogative word such as 何, どこ, だれ, いつ, or どう. Like かい, it is primarily used by male speakers and conveys a warm, approachable tone. だい typically attaches after a noun or na-adjective (which would normally end in だ) at the end of the sentence, effectively replacing the plain だ. It is most natural in questions structured as ～は何だい, ～はどこだい, etc. Learners sometimes confuse だい with かい; the key distinction is that だい appears specifically with WH-words. だい is limited to casual spoken contexts and would sound out of place in polite or formal speech.

Examples

1. お名前は何だい。 What's your name?
2. 次の休みはいつだい。 When is your next day off?
3. トイレはどこだい。 Where's the toilet?

Usage Guide

Context: spoken, everyday

Tone: friendly

Do Say

• 今日の予定は何だい。
• 一番好きな映画は何だい。
• どこに行きたいんだい。

Don't Say

• 先生、これは何だい。(Using だい toward a superior — too casual and disrespectful) → 先生、これは何ですか。
• 明日来るだい。(Using だい for a yes-no question — だい is for WH-questions only) → 明日来るかい。

Origin & History

だい developed from だ (the casual copula) combined with the interrogative nuance of the い suffix. Like かい, it emerged during the Edo period as part of the softened masculine question-particle system.

Cultural Context

Generation: Adults and elderly

Social background: Universal