ではないか / じゃないか

Straight after a noun or a plain form, ではないか points something out with force: "why, it's…!", "look, …!". With の in front, のではないか offers the speaker's opinion as a question: "isn't it the case that…?", "I think…".

Aimed at a fact the listener ignored, the same form without の is a reproach: "I told you!", "you promised!". Out loud, this is almost always the spoken form じゃないか.

ではないか or のではないか

Without の, the speaker is not asking anything: the fact is in front of them, and ではないか says so with surprise or reproach. With の, the speaker is not sure and offers a guess.

• × 明日は雨が降るではないか → 明日は雨が降るのではないか (for a guess)

After the volitional, as in がんばろう頑張ろうではないか, it rallies people: "let's…". That use is its own point, ようではないか.

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ではないか belongs to writing and to formal or old-fashioned speech. In conversation it becomes じゃないか, or じゃない with a rising voice, and じゃないですか when polite. のではないか becomes んじゃないか or んじゃない, and のではないでしょうか when polite.

Polite じゃないですか also brings up something the listener ought to know already, much like "you know…" in English. じゃん is the most casual form of all.