ellibeiteoga gyedan yeope seolchidoeeosseoyo.

가 is the subject marker. It attaches to a noun ending in a vowel (like 엘리베이터) and tells you that this noun is the subject of the sentence. If the noun ended in a consonant, you’d use 이 instead.

• 설치하다 (to install) is the active verb.
• 설치되다 is its passive form (to be installed).
• 설치되었어요 → 설치되다
  • past tense -었어요 = “was installed” (focuses on the elevator receiving the action).
• If you wanted to say “I installed it,” you’d use 엘리베이터를 설치했어요 (active voice).

• 옆 means “side.”
• Adding 에 marks a static location.
• 계단 옆에 literally means “at the side of the stairs,” i.e. “next to the stairs.”

• 옆에서 often marks the place where an action takes place (“do something from/by the side”).
• Since 설치되었어요 describes a state/result (the elevator now exists there), you use 옆에 for location.

Many Korean nouns don’t need a plural marker when the meaning is clear. 계단 inherently refers to a set of steps, so adding 들 is unnecessary and uncommon.

Yes. Korean often drops the subject when it’s understood from context. You’d still understand “It (the elevator) was installed next to the stairs.”

Korean word order is relatively flexible because of particles. Both
• 엘리베이터가 계단 옆에 설치되었어요
• 계단 옆에 엘리베이터가 설치되었어요
mean the same thing. The choice depends on what you want to emphasize first.