What is the difference in meaning between じゃなかった vs じゃない?

Question

Context:

It was agreed upon that there will be a class meeting at 4.30pm later. However, someone wrote the wrong time on the blackboard. And so, one of the characters replied:

これ４時半からじゃなかったっけ？

My question is, why is じゃなかった used instead of じゃない? Could anyone provide a brief description on the nuance difference between the two?

I understand how じゃない and じゃなかった works when referring to state-of-beings, but when put in a question context, I am unable to comprehend the nuance it has.

Answer 1

I think this た is a modal-た described in the following questions and articles.

• Non conventional usage of the past tense
• Usage of plain i-adjectives or た form (悪かったｖ悪い、良かったｖいい etc）
• Conjugated word + んだ vs nonconjugated word + conjugated んだ
• Wasabi - Another Function of the Ta-form: Discovery and Recall

Saying "4時半からじゃない?" is also perfectly grammatical (and is more neutral in a sense), but saying "4時半からじゃなかった?" means the speaker is trying to recall something and possibly update his existing belief. In other words, with this た, the speaker is showing his surprise that his memory may be wrong.

You cannot use った when there is no prior expectation/knowledge. For example, when someone made a simple calculation mistake like "25×70=1250", you can say "1750じゃない?" but not "1750じゃなかった(っけ)?"

っけ has a similar purpose and it is commonly used with た/だ. But it's optional, as shown above.

Answer 2

日本語

日本語での回答になります。

じゃない＝でない、じゃなかっけ（じゃなかったか）＝ではないか

前者は否定、後者は確認として使っていると思います。

また、約束事の時間には影響されません。

English

じゃない = でない / じゃなかっけ（じゃなかったか） = ではないか

The former would be used as negation, and the latter confirmation.

It is not affected by the time the event takes place.