Learn Hardcore Korean: jeongwone simeul namureul gollasseo. - I chose a tree to plant in the garden.

In Korean, relative clauses (describing which tree we’re talking about) always come before the noun they modify. 심을 is the future-attributive form of 심다 (“to plant”), so 정원에 심을 나무 literally means “the tree that (we) will plant in the garden.”

What role does the particle 에 play in 정원에 심을 나무를 골랐어?

Here, 에 marks 정원 as the goal or location of planting. We’re not choosing in the garden, but choosing a tree destined for the garden. If you meant “I chose it while I was in the garden,” you’d use 정원에서 instead.

Why do we use 나무를 with the object particle 를?

나무를 is the direct object of 골랐어 (“chose”). Even though 나무 is modified by a relative clause, it still functions as the object of “to choose.”

What is the tense and register of 골랐어?

골랐어 is the informal-speech past tense of 고르다 (“to choose”). This is casual, everyday language you’d use with friends or family.

Could I replace 심을 with 심은 here? What would change?

Yes, 심은 나무 uses the past-attributive form, meaning “the tree that (someone) has already planted.” That would change the meaning to picking a tree that’s already planted, rather than deciding which one to plant.

Why is there no subject like 나는 in this sentence?

Korean often omits subject pronouns when they’re clear from context. Since you’re talking about your own action of choosing, 나는 is unnecessary and sounds more natural dropped.

How would I say this in a more polite or formal style?

You can change the ending -었어 to -었습니다 for a formal tone:
정원에 심을 나무를 골랐습니다.

Can I split this into two sentences if I’m not comfortable with relative clauses?

Yes. For example:

나는 나무를 골랐어.
그리고 그 나무를 정원에 심을 거야.
This avoids the single complex sentence at the cost of sounding less concise.