Difference between あったんだった and あったんだ

Question

A「お昼休みなのに忙しそうだね」

B「午後から会議があるんだ」

A「ずいぶんゆっくりしてるけど、大丈夫？」

B「あっ、そうか！今日は午後から会議があったんだった」

A「今日一緒にご飯に行かない？」

B「ごめん、今日は彼女との予定があるんだ」

B「ごめん！さっき外食する約束をしたけど、今日は別の予定があったんだ」

Here are two dialogues. What would be the difference between あったんだった and あったんだ?

Answer 1

In such cases, there are two relevant differences to consider:

• is a piece of information known to the speaker or not?
• who does the information come from?

In the first scenario, in which two co-workers talk about a conference, B already knew that a conference was to take place on that particular day. They simply seem to have forgotten and are now re-establishing this knowledge. In that case, the original information about the conference came from A and is now re-stated by B.

In the second scenario, the information that B already has plans is new information to A. Therefore, it is in the present tense, as B is offering this new piece of information to A just now.

Answer 2

This use of た is called modal-ta or 発見のタ. There are a number of questions related to this on this site.

• Non conventional usage of the past tense

  "fixing (or updating) one's previous knowledge/expectation" seems to be the key

• Usage of plain i-adjectives or た form (悪かったｖ悪い、良かったｖいい etc）

  It is used when one's expectation has just matched or failed to match reality.

• Conjugated word + んだ vs nonconjugated word + conjugated んだ

In Dialog 1, B is updating his own knowledge, and that's why the sentence ends with the modal-ta. But 会議があったんだった sounds a little redundant to me. 会議があるんだった is equally correct and probably more common.

In Dialog 2, B is trying to update A's knowledge by telling what is already known to B, so the sentence must not end with the modal-ta. 別の予定があるんだ and 別の予定があったんだ are equally correct, but the latter contains a modal-ta before the end of the sentence to update A's knowledge; it implies "(I know) this is not what you've expected".