You use this to explain something. It tells why something is true. Or it adds more details.
Verb [dictionary form] + のだPrimary
い-Adjective + のだ
な-Adjective + な + のだ
Noun + な + のだ
You use this to explain something. It tells why something is true. Or it adds more details.
「どうして傘を持っているのですか。」「午後から雨が降るのだと、天気予報で言っていたからです。」
"Why are you carrying an umbrella?" "Because the weather forecast said that it's going to rain this afternoon."
彼が急に会社を辞めたのは、新しい事業を始めるためだったのだ。
The reason he suddenly quit the company was that he was going to start a new business.
この問題は、そう簡単には解決できないのだ。
The thing is, this problem cannot be solved so easily.
なるほど、君がいつも楽しそうなのは、充実した毎日を送っているからなのだな。
I see, the reason you always seem so happy is that you're living a fulfilling life every day.
昔はこの辺りも静かな場所だったのだが、今ではすっかり変わってしまった。
This area used to be a quiet place, but now it has completely changed.
The phrase "のだ" is more formal. It is often used in writing. It can sound stiff in casual talk. "んだ" is a shorter, more common way to say it. People use "んだ" a lot in everyday speech.
Don't use this when you just state a fact. For example, 'I am a student' is '私は学生です'. When you use 'のだ' with a noun or a な-adjective, you must add 'な'. For example, '学生なのだ' (It's that I am a student). You cannot say '学生だのだ'. This is also true for '静かなのだ' (It's that it is quiet). But if it's past tense, like '学生だった' (was a student), you don't need 'な'. You can say '学生だったのだ'.
Use this when you want to explain something. It shows why something is happening. It helps others understand the situation better.
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