The Auxiliary Verb

The usage のだ and んだ can be for:

1. Expressing a reason or explanation
2. Emphasis
3. To express what should be

Expressing a Reason or Explanation

大地だいち	明日あしたの飲のみ会かいに行いけないの？
Daichi	You can’t make it to the drinks tomorrow?
雄介ゆうすけ	月曜日げつようびは試験しけんがあるんだ。
Yusuke	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.

月曜日げつようびは試験しけんがあるので、飲のみ会かいに行いけない。

I’ve got an exam on Monday so I can’t go.

近所きんじょの人ひとが夜間やかんを通とおして音楽おんがくをかけたので、今日きょうは眠ねむい。

Today I’m tired because my neighbour played music all night long.

アナ	眠ねむそうだね。
Anna	You look sleepy.
肇はじめ	近所きんじょの人ひとは夜間やかんを通とおして音楽おんがくをかけたんだ。
Hajime	My neighbour was played music all night long.

It is also used at the beginning of explanations…

宿題しゅくだいをしたんですが、持もってくるのを忘わすれました。

I did my homework but forgot to bring it.

彼かれに電話でんわしたんだけど、出でなかった。

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.

この手紙てがみをイギリスに送おくりたいんですが。

I want to send this letter to the UK.

ちょっと聞ききたいことがあるんですが。

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?”

For Emphasis

何なにしてるんだ？！

What on earth are you doing?!

それは嘘うそなんだ！

That’s a lie!

Remember that with nouns and na-adjectives we need a な before the のだ.

To Express What Should Be

何なにをやっても、ベストを尽つくくすのだ。

You should do your best.

言いい訳わけするんじゃないよ！

You shouldn’t be making excuses.

マイク	昨日あしたの夜よるは帰かえりで自転車じてんしゃから落おちて怪我けがしたんだよ。
Mike	Last night I fell off my bike on the way home and hurt myself.
沙織さおり	お酒さけを飲のむんじゃなかったね。
Saori	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”.