じゃないか

1. 1Expresses the speaker's surprise, realization or assertion while inviting the listener to agree ("isn't it!", "why, it's…").Ex. 1
2. 2After a verb's volitional form, strongly urges or invites someone to act together ("let's…!").Ex. 2
3. 3Reproaches or reminds the listener of something they should already know ("I told you, didn't I!").

Used in casual conversation, usually with falling intonation, to present something as obvious or newly noticed and to pull the listener into agreement. The volitional use gives a rousing, somewhat emphatic tone to a proposal. Compared with ね, it is more forceful and assertive, and its more formal counterpart is ではないか.

• ではないかN4ではないか is the formal version of じゃないか; the two are interchangeable for confirming a discovery and for inviting with the volitional form, but ではないか leans written/speech style. It also often appears as のではないか to express a tentative conjecture.
• だろうN5だろう mainly expresses conjecture, but with rising intonation it seeks the listener's agreement, close to じゃないか's confirming and reproachful-reminder uses. However, only じゃないか can express surprise at a discovery or invite with the volitional form.
• ではあるまいかN1ではあるまいか is written style, expressing a tentative conjecture or, after べき, a subtle suggestion, with の required after the verb; じゃないか is colloquial, more assertive, and also covers exclamation, reproach, and invitation. For conjecture they are partly interchangeable, but the register and tone change.
• とはN1とは overlaps only with the surprise-discovery use: とは is more written, emphasizes 'I never expected it,' and is followed by 思わなかった or leaves the rest unsaid, while じゃないか is more colloquial and carries a tone of confirming with the listener. とは has no inviting or reproachful meaning.
• ねN5ね has a soft tone and assumes the listener already agrees; じゃないか is stronger when exclaiming or confirming, often implies surprise, and has a casual, masculine flavor. 行こうね can also serve as an invitation, but is less compelling than じゃないか, and ね has no reproachful use.

ではないか is the more formal or written version of じゃないか, ね seeks agreement more softly, and だろう marks conjecture or assumption rather than emphatic assertion. ではあるまいか is a literary expression of tentative conjecture ("might it not be…"), and とは expresses shocked surprise at a fact ("to think that…") rather than seeking the listener's agreement.