에 앉아 있었어

was sitting on/at

→ 에
phrase Uncommon #10,204
was sitting on/at Used informally to say that someone or something was sitting in a particular place. 에 marks the location, 앉아 means “sit,” and 있었어 indicates a past ongoing state.
KO: 나는 창가에 앉아 있었어.
EN: I was sitting by the window.

정원에 앉아 있었어.

Tom was sitting in the garden.

어둠 속에 앉아 있었어.

Tom was sitting in the dark.