じゃないかIsn't it?, Is it not?

じゃないか and ね both seek agreement at the end of a sentence, but they have different nuances and levels of politeness. じゃないか is casual and assertive, often strongly insisting on an opinion, and can feel confrontational, making it best suited for use among close friends. In contrast, ね is softer and used widely to seek gentle agreement or confirmation in various contexts.

でしょう, in addition to expressing conjecture, can also seek confirmation or agreement from the listener (e.g., 面白いでしょう = 'Interesting, right?'). じゃないか is more forceful and casual, implying the speaker strongly believes something and expects the listener to agree (e.g., 面白いじゃないか = 'Interesting, isn't it!'). Note that でしょう is polite, while じゃないか is very casual.

じゃない follows a noun or な-Adjective to express 'is not' or 'are not.' じゃないか, with the addition of か, changes the meaning to 'Isn't it...?' as in 本じゃない ('It isn't a book') vs. 本じゃないか ('It's a book, isn't it?') While じゃない is used only after nouns or な-Adjectives, じゃないか can follow い-Adjectives and verbs as well.

だろう, in addition to expressing conjecture ('It's probably…'), can seek confirmation or agreement from the listener (e.g., 面白いだろう = 'It's interesting, right?'). じゃないか is more forceful and assertive, suggesting the speaker strongly believes something and expects agreement (e.g., 面白いじゃないか = 'It's interesting, isn't it!'). Both are casual forms and should be avoided in polite conversation.