Learn Hardcore Korean: deungsanhal ttae sajineul jjigeoyo.

등산 means “hiking” or “mountain climbing.” To say “when (I/you/we) hike,” you attach 하다 (to do) to make 등산하다, then put it into the attributive form 할 and add 때 (time/when). So 등산할 때 literally means “the time when (we) hike,” i.e. “when (we) go hiking.”

Why do we need 할 in 등산할 때? Isn’t 등산 already a noun?

Even though 등산 is a noun, to express the action “when hiking” you must use the verb 하다 in its attributive form 할 before 때. Think of it as “the time of doing hiking.” Without 할, you can’t grammatically attach 때 to a standalone noun.

Can I say 등산을 할 때 instead of 등산할 때? What’s the difference?

Yes, 등산을 할 때 is also correct. Here you explicitly mark 등산 as the object of 하다 with 을. In everyday speech, Koreans often drop that object particle in compound nouns (등산할 때), but 등산을 할 때 is a more “textbook” way and adds a slight emphasis on 등산 as an action you do.

Why is 사진을 followed by 을? Could I just say 사진 찍어요?

In standard Korean, transitive verbs like 찍다 (“to take [a photo]”) require their object to be marked with 을/를. So 사진을 찍어요 is the grammatically complete form. In casual speech, especially in conversation, Koreans sometimes drop particles, so you might hear 사진 찍어요, but using 을 makes the sentence clearer and more formal.

Why does the verb 찍어요 come at the end? Is that always the case in Korean?

Yes. Korean follows a Subject–(Indirect Object)–Direct Object–Verb order (SOV). Modifiers (like 등산할 때) come before the verb. That’s why 사진을 찍어요 (“take a photo”) places the object 사진을 before the verb 찍어요 at the very end.

What level of politeness is conveyed by the ending -어요 in 찍어요?

The ending -어요 is the polite informal style (합쇼체). It’s used with people you’re not extremely close to—like acquaintances or strangers—to show respect without sounding too formal. If you want to be more formal, you’d say 찍습니다; if you’re with close friends or younger people, you might drop to the casual 찍어.

How do I change 찍어요 to the past tense?

You replace the -어요 ending with -었어요 attached to the verb stem 찍-. That gives 찍었어요. So “I took pictures when I went hiking” is 등산할 때 사진을 찍었어요.

How can I turn this into a question: “Do you take pictures when you go hiking?”

Just keep the same word order and raise your intonation, or add a question particle if you like. Example:
등산할 때 사진을 찍어요?
This is a polite yes/no question in the same polite informal style.

What’s the difference between using 때 and 동안 for time expressions?

~할 때 means “when (doing something),” referring to a point or period at the moment the action happens.
~하는 동안 means “while (doing something)” or “during the time that (something) continues,” focusing on the entire duration.
So 등산할 때 사진을 찍어요 = “I take pictures when I hike” (at that moment), whereas 등산하는 동안 사진을 찍어요 = “I take pictures during the time I am hiking,” emphasizing the ongoing activity.