NO DA AND NO DESU (のだ / のです)
NO DA (のだ) (or NO DESU in the polite form) is used to provide an explanatory tone to the statement. In other words, the presence of this structure at the end of the statement shows that you are trying to explain something, or to have an explanation if it is a question.
This structure is placed after a verb in the plain form (dictionary form or plain form of the past -TA), or after an adjective in い. When it’s an adjective in な or a noun, you need add な between this structure and the noun (なのだ).
のだ is often contracted when spoken んだ (んです in the polite form). It can also be shortened to a simple の in a question (the famous interrogative の).
Look at this first example:
|Chuugokugo o benkyou suru tsumori na n desu ka.|
|Do you intend to learn Chinese?|
In the example above, you can imagine that you see someone reading a book of Chinese, and you want to know a little more, have an explanation of why he’s learning Chinese, for example. Without んです in the sentence, the question would be more general without specific context; we simply pose a question to a person who is perhaps not at all interested in the Chinese language. The relationship between the individuals is therefore more direct and less personal.
It’s mainly a matter of understanding the nuance between information and explanation. If you don’t use のだ, you simply provide information very objectively. With のだ, there is a more subjective side; the speaker is involved more personally in an explanation.
The person who answers this question is very likely to use のです at the end of his/her statement to stay in the tone of the explanation. In linguistics, it’s called the explanatory mode: のです has no meaning in itself, but modifies / modulates the meaning of the statement.
Depending on the context, the sentence can be translated in different ways, but always remains in this explanatory mode. Let’s say you were to have called back a friend and you didn’t. Your friend asks you why you didn’t call. You’ll then explain yourself, and even justify yourself.
|Keitai denwa ga koshou shita n desu.|
|(It’s because...) My mobile stopped working.|
In such situations, のだ could be translated by phrases such as “it’s because...,” “the fact is that...,” “you see...,” “actually...” to give a reason.
|Kyou wa kibun ga warui n da.|
|(Actually...) Today, I’m not feeling well.|
Depending on the situation and the intonation, the person asking the question asks for an explanation. Doing so, のだ can bring out anger, curiosity, astonishment, etc. Again, without のだ, the sentence would be neutral and objective. The statement in that case would leave no emotion on the semantic level, only through the intonation if necessary.
|Ittai nani o shite iru no?|
|But damn, what are you doing?|
Here you feel the exasperation from the speaker who wants to know why he has to wait so long.
|Nihongo mo hanaseru n desu ka!|
|You speak Japanese, too?!|
This time, the speaker expresses a certain surprise and curiosity.
For adjectives in な and nouns, use なのだ and なのです, which be contracted in なんだ and なんです when speaking.
|Kyou no tesuto wa rakushou na n da yo.|
|This morning’s exam is too easy!|
|Kuruma ga hitsuyou na n desu.|
|I need a car.|
To finish, here are the different inflections of んだ and んです.
|Plain form of verb / い adjectives||Noun / な adjectives|