Learn Hardcore Korean: jidoreul bogo ttokbaro georeoyo. - I look at the map and walk straight.

보고 here is the connective form of the verb 보다 (“to look”) plus -고, which links two actions. In other words, 지도를 보고 means “look at the map and …,” implying “after (or while) looking at the map.” So the sentence literally says, “Look at the map, then walk straight.”

Why is the verb 걷다 conjugated as 걸어요 instead of 걷어요?

걷다 is a ㄷ-irregular verb. When the stem ends in ㄷ and a vowel-initial ending (like -어요) follows, the ㄷ changes to ㄹ. Hence:
• 걷 + 어요 → 걸어요

What does 똑바로 mean, and what exactly does it modify?

똑바로 is an adverb meaning “straight” (in direction) or “properly.” Here it modifies 걸어요, telling you how to walk. In context, it most naturally means “go straight” rather than “walk properly.”

Could 똑바로 걸어요 ever mean “walk properly” instead of “walk straight”?

Yes, 똑바로 걸어요 can mean “walk with good posture” or “walk properly,” especially if the context is about etiquette or posture. But when giving directions (with 지도를 보고), it’s understood as “walk straight.”