Learn Hardcore Korean: gapjagi biga wayo. - It suddenly starts to rain.

-가 is the subject marker, highlighting 비 (rain) as the performer of the action. Using -는 would make rain the topic (“As for rain…”), which shifts the nuance and isn’t needed when you simply want to say “rain is coming.”

Why do we say 와요 instead of 오어요 or just 오다?

오다 is an irregular verb in Korean. In the polite present tense, it contracts from what you might expect (오어요) to 와요. We don’t use 오다 in its dictionary form in a sentence like this.

Why do Koreans use 오다 (“to come”) for rain? Wouldn’t 내리다 (“to fall”) make more sense?

In everyday Korean, weather phenomena like rain or snow “come” rather than “fall.” So 비가 와요 is the normal spoken expression. 비가 내립니다 or 비가 내려요 both mean “it’s raining,” but 내리다 sounds more formal or literary.

What level of politeness is 와요, and how can I make the sentence more formal or more casual?

와요 is the standard polite level (해요체) used in most conversations.
• To make it more formal (하십시오체): 갑자기 비가 옵니다.
• To make it casual/informal (해체): 갑자기 비가 와.

Can I omit 비가 if it’s obvious from context?

Yes. Korean often drops known subjects. If you’ve already been talking about rain, you can simply say 갑자기 와요 and listeners will understand.

Is the word order flexible? Could I say 비가 갑자기 와요?

Absolutely. Korean word order is relatively free. 비가 갑자기 와요 and 갑자기 비가 와요 mean the same thing; you’re just shifting emphasis slightly by placing the adverb in a different spot.

How do I change 와요 to past tense to say “It suddenly rained”?

Use the past polite form of 오다, which is 왔어요. So you get 갑자기 비가 왔어요.

How can I express that it’s raining heavily or that it suddenly started pouring?

• Heavily: 갑자기 비가 많이 와요
• Torrentially: 갑자기 폭우가 와요 (폭우 = torrential rain)
• Suddenly started: 갑자기 비가 오기 시작했어요 or 갑자기 비가 내리기 시작했어요.