〜んだった goes after the dictionary (plain) form of a verb in a sentence. This can be used for past or “future” tense.
Although it is past tense, we still use the plain (present) tense of the verb and just add on んだった。This is applicable to all sections of this post.
When Talking About Yourself
この辺りは治安が悪い。もっと治安のいいところに引っ越すんだった。Kono atari wa chian ga warui. Motto chian no ī tokoro ni hikkosundatta.
This neighborhood is unsafe. I should have moved to a nicer area. (lit. this area is not secure. Should have moved to more secure/good area.)
When Talking To Someone Else
you usually な or ね at the end, depending on your gender, male and female, respectively–
もっと早く出かけるんだったね。Motto hayaku dekakerundatta ne(na).
You should’ve left earlier. (lit. should’ve left more early.)
This is the girl version, since the speaker (myself) is a girl.
Forgetting To Do Something
Past tense
おお、卵を買うんだった。Oo, tamago o kaundatta.
Ugh, I forgot to buy the eggs. (lit. ugh, was supposed to buy eggs.)
Future tense
わっ、明日テストを受けるんだった！wa-, ashita tesuto o ukerundatta!
Ugh, I forgot that there’s a test tomorrow! (lit. ugh, tomorrow was supposed to take test.)
Future tense in Japanese is completely dependent on context since there technically is no future tense, but it’s obvious from this example that the speaker is talking about the future (tomorrow).
-Ivey
