Learn Hardcore Korean: gakkeum naneun jumare sane deungsanhareo gayo. - Sometimes I go hiking in the mountains on weekends.

산에 “to the mountain” marks the destination with the particle -에.
등산하러 uses the purpose form (verb stem 등산하-
- -러) meaning “in order to hike.”
  Put together, 산에 등산하러 가요 literally means “I go to the mountain in order to hike.”

Why are both 가끔 and 주말에 used? Don’t they overlap?

They serve different functions:

가끔 = “sometimes,” expresses frequency.
주말에 = “on weekends,” indicates time frame.
So 가끔 주말에 means “on some (but not every) weekend.”

What role does 나는 play here? Can I drop it?

나는 is the topic marker (“as for me”). In Korean, topics and subjects are often dropped if context is clear.

With 나는, you explicitly set “I” as your topic or contrast.
Without 나는, 가끔 주말에 산에 등산하러 가요 is perfectly natural once it’s clear you’re talking about yourself.

Why does 주말 take -에? I thought -에 was for locations.

The particle -에 attaches to time nouns (days, hours, seasons) to mean “at/on/in.”

주말에 = “on the weekend.”
It works exactly like 학교에 “to school,” but with a time instead of a place.

Why is it 등산하러 가요 instead of just 등산해요?

등산하러 가요 = “go to hike,” emphasizing movement to the mountain for the purpose of hiking.
등산해요 = “do hiking,” implying you’re already at the hiking spot.
Use -러 가다/오다 when you want to express going somewhere to do that action.

What does the -요 ending in 가요 convey? Can I say 간다?

가요 is the polite present tense form (해요체).

간다 is the plain/present declarative (written narration or close friends).
가 is informal low-polite (반말).
Choose based on your relationship and level of politeness.

Why is the word order 가끔 나는 주말에 instead of 나는 주말에 가끔?

Korean is flexible, but ordering adverbs upfront (가끔) gives them extra emphasis.
You can also say 나는 주말에 가끔 산에 등산하러 가요, which is equally correct—just with a slightly different emphasis.

Why is it 산에 and not 산에서?

산에 가요 = “go to the mountain” (에 marks direction).
산에서 would mark the place where an action happens (“at the mountain”).
Since you’re expressing movement to the mountain, you need -에.

Could I say 주말에 가끔 instead of 가끔 주말에? Which is more natural?

Both are natural.

가끔 주말에 front-loads the frequency.
주말에 가끔 front-loads the time.
Choose whichever nuance (frequency- vs time-emphasis) you prefer.

What’s the difference between using 나는 and 내가 here?

나는 (topic particle) sets “I” as the topic. Often used for general statements or contrasts.
내가 (subject particle) emphasizes “I” as the doer right here.
In everyday speech, 나는 … 가요 is more natural when you’re just telling what you do.