のだ — The Story Behind the Statement

Compare these two sentences:

A. 昨日会社を休みました。 → I took the day off yesterday. B. 昨日会社を休んだのは、頭が痛かったからです。 → The reason I took the day off yesterday is that I had a headache.

A simply states a fact. B, with のだ, is explaining the reason — "You might be wondering why I was absent. It's because I had a headache."

The core function of のだ (commonly said as んだ/んです in spoken Japanese) is: providing the backstory or explanation behind a known fact.

Three Main Uses

1. Explaining a reason — Why?

The most common use. The listener has observed something, and you use のだ to explain the reason behind it.

• どうして遅れたんですか。 — 頭が痛かったんです。 → Why were you late? — I had a headache.

• 目が赤いですよ。 — 昨日あまり寝なかったんです。 → Your eyes are red. — I didn't sleep much last night.

2. Rephrasing — In other words

Used to restate something in a different way.

• 彼は毎日ジムに通っている。つまり、健康に気をつけているのだ。 → He goes to the gym every day. In other words, he's health-conscious.

3. Discovery — So that's how it is

Realizing something you didn't know before.

• あ、外は雨が降っているんだ。 → Oh, it's raining outside. (Just noticed)

• そうか、君が犯人だったんだ。 → I see, so you were the culprit all along. (A sudden realization)

Spoken Forms

In everyday conversation, people almost never say 「のだ」 — they say 「んだ」 or 「んです」 instead.

• どうしたんですか？ → What happened?
• ちょっと熱があるんです。 → I have a bit of a fever.

Self-Test

Q1. A coworker sees you wearing a mask and asks what's wrong. You reply: 「風邪を引いた＿＿です。」

A. の   B. こと   C. もの

Q2. What use is 「あ、もう12時なんだ。」?

Summary

• のだ/んです = providing the backstory or explanation behind a fact
• Three uses: explaining a reason, rephrasing, and discovery
• Spoken forms: んだ/んです; written forms: のだ/のです
• Explaining a reason is the most common use — the listener observes something, and you explain the why behind it