[yuriyuri]

～んだ can also be used as a command form.

eg:
忘れるんだ！ - Forget it!
逃げるんだ！ - Run!
etc.

Edit:
I should probably mention that I think it only gets used as a command form on the end on plain verbs.
Don't take my word for that but that's how I've always seen it used.