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?