Learn Hardcore Korean: gyedaneul ollagal ttae josimhaeyo. - Be careful when going up the stairs.

In Korean, 올라가다 (“to go up, to ascend”) is often treated as a transitive verb taking the stairs as its object. That’s why we say 계단을 올라가다 (“to ascend the stairs”) with the object particle 을.

If you said 계단에 올라가다, it would mean “to go onto the stairs” (intransitive), focusing on entering the staircase rather than climbing it.
Using 계단을 emphasizes the action of climbing the stairs themselves.

How do we form 올라갈 때 from 올라가다, and why not 올라가는 때?

To express “when doing X” in Korean, you use the verb stem + ㄹ 때 (for actions yet to occur or general circumstances). Here’s the breakdown:

Base verb: 올라가다 (“to go up”)
Remove 다: 올라가
Future/adverbial suffix: + ㄹ → 올라갈
Time noun 때 (“time, when”) → 올라갈 때 (“when (you) go up”)

Why not 올라가는 때?

올라가는 때 (using 는 때) could imply “the time during which (you) are going up,” focusing on the ongoing process.
올라갈 때 is more natural for general warnings or advice: “when you’re about to/are going to go up.”

Where is the subject in 계단을 올라갈 때 조심해요? Who are we talking to?

In Korean, the subject is often dropped when it’s clear from context. Here, the implied subject is you.

Full form (very explicit): (당신이) 계단을 올라갈 때 조심해요.
Because it’s a general warning to anyone listening, Korean just omits 당신이 and leaves it understood.

In 조심해요, why does 조심 behave like a verb? And what does 해요 do here?

조심 is originally a Sino-Korean noun meaning “caution” or “care.”
Adding -하다 turns many Korean nouns into verbs: 조심
- 하다 → 조심하다 (“to be careful”).
해요 is the polite present tense form of 하다. So 조심해요 literally means “(I/you) do caution,” i.e. “be careful.”

Is 조심해요 actually an imperative (“be careful”) or just a statement (“you are careful”)?

Though grammatically it’s the present tense polite form, in spoken Korean context it functions as a polite command or advice. In other words, 조심해요 = “Please be careful.” The tone and situation tell the listener it’s a warning, not a neutral statement.

What’s the difference between 조심해요 and 조심하세요?

Both are polite, but:

조심해요 uses the plain polite ending -해요, suitable among friends or peers in a polite register.
조심하세요 uses the honorific/command ending -(으)세요, which is more formal or respectful—common when speaking to strangers, elders, or in customer‐service contexts.

Can I drop the particle 을 and say 계단 올라갈 때 조심해요?

Yes. In casual spoken Korean, dropping particles (especially 을/를) is very common:

계단 올라갈 때 조심해요 sounds natural in everyday speech.
In formal writing or careful speech, include 을 for clarity: 계단을 올라갈 때 조심해요.