Learn Hardcore Korean: jeongwone simeossdeon kkotdeuri beolsseo pieosseo. - The flowers I planted in the garden have already bloomed.

-었던 marks a past action that the speaker has in mind and links it to the current situation.
It turns the verb into a modifier for 꽃들 (“the flowers that I had planted”).
Compared to 심은, 심었던 often carries a nuance of recalling or emphasizing that this was done before and now its result matters. You could say both 정원에 심은 꽃들 and 정원에 심었던 꽃들, but the latter sounds slightly more reflective (“the ones I specifically planted back then”).

Why is the particle 에 used after 정원? Wouldn’t 에서 (“from/in”) be more natural for “in the garden”?

In Korean, 정원에 심다 is the standard collocation for “to plant in a garden.” Here’s why:

에 can mark the target location or destination of an action (“put/plant something at X”).
에서 emphasizes the location where an action is taking place (“I’m doing something at X”).
Since planting focuses on placing the flower into the garden’s soil (the target), 에 is preferred. Saying 정원에서 심었다 is grammatical, but it shifts the nuance to “the garden is where I did the planting,” rather than “inside the garden.”

What’s the role of the plural marker 들 in 꽃들이? Could I just say 꽃이?

들 simply makes it clear you’re talking about multiple flowers.
꽃이 could also work; Korean often omits explicit plurals when context is clear.
Using 꽃들이 emphasizes “those flowers (all of them) that were planted.” It’s more natural if you imagine many blooms popping up.

What nuance does 벌써 carry here?

벌써 means “already,” but with a shade of surprise or that something happened sooner than expected. In 벌써 피었어, the speaker implies, “Wow, they’ve bloomed so quickly!”

Why is the verb 피었어 in the past tense? Wouldn’t blooming be ongoing, like 피고 있어?

피었어 (피다 + -었어) states that the action is complete: “they have fully bloomed.”
피고 있어 would mean “they are in the process of blooming” (still unfolding).
Since the flowers are already open, the perfective/past marker -었어 is more appropriate.

What level of politeness is 피었어? How would I say this politely?

피었어 is 반말 (informal/casual) speech, used among close friends or younger people.
Polite versions:
• 피었어요 (standard polite)
• 피었습니다 (formal)

Could I use 이미 instead of 벌써, and what’s the difference?

이미 also means “already,” but it’s a neutral statement of fact.
벌써 adds the speaker’s sense of surprise or impatience (“so soon!”).
You could say 이미 피었어, but you’d lose that extra nuance of unexpected speed.