jeoneun jumalmada saneseo deungsanhaeyo.

-마다 means “every” or “each time.” When you attach it to a noun, it indicates repetition at every instance of that thing.
• 주말마다 = “every weekend”
• 한 시간마다 = “every hour”
• 한 달마다 = “every month”

The particle -에서 marks the location where an action takes place. Since hiking is an activity, you use 산에서 (“at/on the mountain”).
• 산에서 등산해요. = “I hike on the mountain.”
In contrast, -에 often marks a static location or direction:
• 산에 가요. = “I go to the mountain.”

등산해요 is the verb form meaning “(I) do mountain climbing/hiking.”
• 등산 by itself is a Sino-Korean noun meaning “mountain climbing.”
• By adding 하다 (“to do”), you turn it into a verb: 등산하다 → 등산해요 (present tense, polite).

No, Korean often omits the subject when it’s clear from context. If you’re already talking about yourself, you can simply say:
• 주말마다 산에서 등산해요.
Adding 저는 just makes it explicit or adds a slight contrast (“As for me, I hike…”).

• -는/은 is the topic marker, indicating what or whom you’re talking about. It often sets up contrast or context.
• -이/가 is the subject marker, focusing on the new or emphasized subject in the sentence.
  Here, 저는 sets “me” as the topic (“As for me…”).

Use the formal declarative ending -습니다 instead of -해요:
저는 주말마다 산에서 등산합니다.