Explanatory feeling

のです and its derived forms, のだ/んです/んだ, are used to add more information in a sentence. You will see them when we are asking for a reason or information, or when we want to put emphasis on a reason. You might see the form んです or んだ more often than the other ones. Let's see how we use them in sentences!

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Verb + んです

Adjective い + んです

Adjective な (without な) + なんです

Noun + なんです

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This suffix is usually found at the end of sentences, and it will be used to fill in the gap of understanding between the speaker and the listener. Thus, you will find this expression a lot when people are trying to solve misunderstandings. It brings an information that will add an understanding on the listener's part.

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For example, let's say that you are walking down the street with you friends when someone asks for your help. The person only speaks Spanish, but no worries, because you can speak Spanish too! So, you help the person, talking to them in Spanish. The person, thanking you, goes on their way, and your friends say: 'you are pretty good at speaking Spanish!' And then, you say: 'it is because I spent 5 years in Spain'.

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You are adding information to explain the fact that you can speak Spanish. 'It is because...' will roughly be translated as のです. 

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You will see it sometimes with the term 実は (じつは).

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実は東京に生まれたんです。

Jitsu wa Toukyou ni umaretandesu.

It's because I was born in Tokyo.

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Most of the time, the term 実は will come with のです. Simply saying 実は東京に生まれた still gives the information, but this might come off as unnatural and as a random statement. Adding のです creates a natural flow for you to complete the context by saying the reason why you can speak Japanese for instance is because you were born in Tokyo. 

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の in のです is a nominalizer. With this construction, the ます-form is not possible.

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X 実は東京に生まれましたんです。X

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O 実は東京に生まれたんです。O

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X 本を読みますんだ。X

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O 本を読むんだ。O

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The explanatory feeling in Japanese cannot be translated in English, it only represents a change in nuance in the sentence. 

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You can prompt other people for more information by using の at the end of your question. You can include the question particle か or not.

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何するの？

Nani suru no?

What are you doing?

何をしているんですか？

Nani o shiteirundesu ka?

What are you doing?

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Other examples:

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なんでゲームをしに来なかったのか？

Nande geemu o shini konakatta no ka?

Why didn't you come and play games?

ごめん。かぜをひいてちゃったんだから、家で休んだ。

Gomen. Kaze o hiitechattanda kara, ie de yasunda.

Sorry. I caught a cold, so I rested at home.

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のです/んです/のだ/んだ for emphasis

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This expression can also be used to put emphasis. 

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それは嘘なんだよ！

Sore wa uso nanda yo!

That is a lie!

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獣医になるのは夢なんだ。

Juui ni naru no wa yume nanda.

It is my dream to become a veterinarian.