kyuu ni samuku narimasita.

It suddenly became cold.

Breakdown of kyuu ni samuku narimasita.

• 寒さむい: cold
• なる: to become
• 急きゅう に: suddenly

Questions & Answers about kyuu ni samuku narimasita.

What does 急に mean here, and why is に used?

急に (きゅうに) means “suddenly” or “abruptly.”

• 急 is a な-adjective (形容動詞).
• To turn a な-adjective (or many nouns) into an adverb, you add に:
  • 急だ → 急に (“be sudden” → “suddenly”)

Why is 寒い changed to 寒く before なりました, and what does the ～くなる pattern mean?

This is the ～くなる pattern, which expresses “to become …”:
1) Take an い-adjective like 寒い.
2) Drop the final い, add く → 寒く.
3) Attach the verb なる (“to become”).
4) In the sentence, なりました is the polite past form of なる.
So 寒くなりました means “became cold.”

Could I just say 寒くなりました without 急に? How does the nuance change?

Yes.

• 寒くなりました = “It became cold.”
• 急に寒くなりました adds the sense of abruptness: “It suddenly became cold.”

Is 急に寒くなりました the same as 突然寒くなりました?

They’re very similar—both mean “suddenly.”

• 急に is more common in everyday speech.
• 突然 (とつぜん) feels a bit more formal or literary, often in writing or news.

Why is there no subject in this sentence? Can I add one?

Japanese often omits the subject when it’s understood from context. Here, it’s obviously talking about the weather.
You can make it explicit:
• 天気が急に寒くなりました。

How would you express this in different tenses or levels of politeness?

• Casual past: 急に寒くなった。
• Casual present/future: 急に寒くなる。
• Polite present/future: 急に寒くなります。
• Polite past: 急に寒くなりました。 (original)

What kind of verb is なる, and why don’t we add です after it?

なる is an intransitive verb meaning “to become.” It already functions as the predicate, so you attach polite endings (-ます, -ました) directly. You don’t use です after it because なる is not a noun or adjective.

Is 急激に the same as 急に? When might you use one over the other?

• 急に – common, neutral, spoken style.
• 急激に (きゅうげきに) – more formal/literary, emphasizes a sharp or steep change (e.g. “stock prices fell sharply”).