The 終助詞（しゅうじょし） (sentence-ending particle)
かい is a grammar structure that is used in Japanese to highlight a question. Despite commonly being thought of as one word, it is actually the combination of
か (the sentence-ending particle) and い, another sentence-ending particle that is primarily used by men (in a similar way to
ね and
よ).
かい will usually be used directly after the word/phrase that it is highlighting. In the case of verbs and
い-Adjectives,
の may be inserted before
かい. For
な-Adjectives and nouns,
なの may be inserted directly before it. However,
の (or
なの) are frequently omitted, due to how casual
かい is.
When it is not omitted,
の (or
なの) may be shortened to ん (or なん）. This is in the same way that
の and
なの would behave in the
のです and
なのです grammar structures.
-
お前（まえ）も一緒（いっしょ）に行（い）きたいんかい？
Do you also want to come with me?
-
大学生（だいがくせい）なんかい。中学生（ちゅうがくせい）だと思（おも）った。
You are a college student? I thought you were a middle schooler.
Although there are some exceptions,
かい will almost always be used to ask simple yes/no questions.
-
勉強（べんきょう）してるのかい？
Are you studying?
Because
かい is very casual, and quite direct, it often sounds like an accusation. Due to this, care should be taken with your tone of voice, so as not to be taken the wrong way.
Outside of かい, the sentence ending particle い is not commonly used in modern Japanese, and therefore will only appear in a few very specific structures (like だい). Despite being similar to ね, い is considered to be fairly ぞんざい ‘rough’, and should only be used with people that you are very familiar with, and never to someone above you.