collapsed; fell down쓰러졌어요 is the polite past form of 쓰러지다, meaning that someone or something fell down or collapsed. 이 is the subject particle, attached directly to a noun ending in a consonant. The exact sequence 이 쓰러졌어요 occurs when the subject noun ends in 이, as in 거신병이 쓰러졌어요 (The giant soldier collapsed).