かい

A sentence-final question particle for casual yes/no questions. It adds a soft, friendly tone and is typically used by men or older speakers when talking to equals or people of lower status.

Used in casual conversation, especially by male or older speakers addressing friends, family, children, or subordinates. It sounds warmer and gentler than a bare plain-form か question. Unlike ませんか, it is a casual question rather than a polite invitation, and unlike ね, it asks for information rather than seeking agreement.

• かN5「か」 is neutral in register: it attaches to 「です」 and 「ます」 to form polite questions with 「ですか」 and 「ますか」, and also appears in questions with a question word and in 「〜か、〜か」 either-or questions. Plain form + 「か」 sounds blunter and stiffer than 「かい」, without its friendly warmth.
• かしらN4「かしら」 is mostly self-directed wondering or a light question that doesn't demand an answer, traditionally feminine in feel, and after a negative form it expresses a hope or request. 「かい」 directly asks the listener whether something is so, is common among men and older speakers, and can also form a rhetorical question implying denial, so the two are generally not interchangeable.
• じゃないかN4「かい」 genuinely asks a yes-no question, and when rhetorical it implies the negative. 「じゃないか」, despite its negative-question form, actually expresses an affirmation or exclamation, can follow the volitional form to make a suggestion, or is used to reproach; it is generally not a real question, so the two cannot be swapped.
• ねN5「かい」 asks when you don't know the answer, or implies the negative as a rhetorical question, while 「ね」 assumes the listener shares your view, seeking empathy or confirming something known or inferred, so they are not interchangeable. 「ね」 can follow polite forms, and after a noun 「だね」 is common.
• ませんかN5「ませんか」 is polite, attaches only to verbs (to the ます form with ます removed), and makes a gentle invitation or inquiry. 「かい」 attaches to plain forms (directly after nouns and な-adjectives in the present tense) for casual or rhetorical questions to people you know well or to juniors; the registers differ, so they shouldn't be swapped.

「か」 is a neutral question particle, and 「かい」 adds a friendly, easygoing tone to it; 「かしら」, mostly used by women, expresses self-directed wondering or speculation or softly and indirectly asks or requests something of the listener (e.g. 「ちょっと来てくれるかしら」), whereas 「かい」 sounds more masculine and direct. 「じゃないか」 is used to confirm, reproach, or draw attention, 「ね」 seeks the listener's agreement rather than simply asking, and 「ませんか」 is a polite suggestion or invitation, the exact opposite in register of 「かい」's casual questions.