かい

かい ends a yes/no question in casual speech and makes it sound warm and friendly. It goes after the plain form; a noun or a な-adjective drops だ before it.

• × 元気だかい → 元気かい

かい is mostly men's speech, and it sounds a little old-fashioned: an older man to a younger person, a parent or teacher to a child. Younger speakers more often ask with の or with rising intonation alone.

かい or だい

かい only asks yes/no questions. A question with a word like "where", "what" or "who" takes だい instead, usually after んだ or straight after the question word.

• × どこに行くかい → どこに行くんだい

かい, か and の

A plain-form question ending in か sounds blunt, even rough. かい softens it, and の is the neutral choice anyone can use. のかい combines the two: it asks about something the speaker has just noticed or heard.