Can んだった and んじゃなかった be used like the past version of んだ and んじゃない?

Question

I know that んだった and んじゃなかった can be used to say "Should have done" and "Shouldn't have done". I am wondering, however, if they can be used as a past version of んだ Take for example this conversation:

A friend: You never spend time with me

You: 忙しいんだ (The thing is, I'm busy)

Now I try with んだった

A friend: You never spent time with me

You: 忙しいんだった (I was busy, that was why)

Is this incorrect or correct?

Much appreciated

Daniel Safari

Answer 1

I'm going to offer a different answer.

I think it does not work the way you think.

んだ is a statement, which works as emphasis of what is being said/thought now. It can not be used the same way in the past.

忙しいんだ。
It's just that I'm busy.

has as past tense

忙しかったんだ。
It's just that I was busy.

whereas

It was just that I was busy.

cannot be expressed in Japanese the same way. 忙しかったんだった is certainly most cumbersome. In English, "It's just that" can be conjugated to "It was just that" to make an explanation about something in the past, which is not how it works in Japanese.

Answer 2

忙しいんだった

I could imagine saying this (to myself) if I accidentally bothered someone, after being told to leave them alone because they were busy.

"it was the case that [he?] is busy"

"(oh, I forgot,) [you] were busy" (and still are)

You can't say something like あ、忙しかった for this purpose -- that implies they're no longer busy.

忙しかったんです

This fits the situation you're describing.