The usage のだ and んだ can be for:
- Expressing a reason or explanation
- To express what should be
Expressing a Reason or Explanation
|You can’t make it to the drinks tomorrow?
|I’ve got an exam on Monday.
This is best understood by thinking of it as a form of ので . The reasoning becomes clearer if we make the sentence complete.
|I’ve got an exam on Monday so I can’t go.
|Today I’m tired because my neighbour played music all night long.
|You look sleepy.
|My neighbour was played music all night long.
It is also used at the beginning of explanations…
|I did my homework but forgot to bring it.
|I called him but he didn’t pick up.
Finally, one extremely useful phrase: ending the sentence with んですが when we want to make a request.
|I want to send this letter to the UK.
|I have something I want to ask you quickly.
We don’t really have direct equivalent in English (it’s kind of like verbally expressing “…” at the end of the sentence as a prod for the listener to take some action for you). In the above examples we want someone to go ahead and say, “Sure, put it on the scales” or “No problem. Go ahead”. The んですが expression is simply a way to omit the obvious “… are you able to do that?” or “… is that okay?”
|What on earth are you doing?!
|That’s a lie!
Remember that with nouns and na-adjectives we need a な before the のだ.
To Express What Should Be
|You should do your best.
|You shouldn’t be making excuses.
|Last night I fell off my bike on the way home and hurt myself.
|You shouldn’t have drunk alcohol.
Note how in the first sentence the んだ is used for emphasis whilst in the second it is used by the speaker to express “should not have”.