Learn Hardcore Korean: keun saneul bomyeon jogeum ginjangdwaeyo. - I feel a bit nervous when I see a big mountain.

-으면 is the conditional suffix attached to verb stems. When you put it on 보다 (“to see”), you get 보면, which means “when/whenever (I/you) see…”. It indicates a general or repeated condition rather than a single past event.

Why is 큰 산 marked with the particle -을?

The verb 보다 is transitive, so it needs a direct object. -을 (or -를 after a vowel) marks the direct object. Here, 큰 산을 literally means “(the) big mountain” as the thing being seen. If you used an intransitive verb, you’d choose a different particle (e.g. 산이 with 이/가).

Why is the subject (e.g. “I”) omitted in this sentence? How do we know who’s doing the seeing?

Korean often drops the subject when it’s clear from context or when making a general statement. In 큰 산을 보면 조금 긴장돼요, the implied subject is “I”, but leaving it out keeps the sentence natural and succinct.

What’s the difference between 긴장하다 and 긴장되다, and why is 긴장되다 used here?

긴장하다 is an active (transitive) verb meaning “to make (someone) nervous” or “to do the act of being tense.”
긴장되다 is the passive/intransitive form meaning “to become nervous” or “to feel tense.”
When talking about your own reaction or feeling, Koreans typically use 긴장되다.