1.고서 1) 고서 connects two or more actions in a sequence. 나는 그 전화를 받고서 밖에 나갔다. I went out after receiving the call. 2) Sometimes, the first action in a sequence can be the reason for the 2nd action. 나는 그 전화를 받고서 많이 놀랐다. I was shocked after receiving the call. 3) Sometimes, the first action can be the means of the second action. 연희는 차를 몰고서 해변으로 갔다. Yoenhee drove a car to the beach. 2.고서야 1) only after 나는 그 온라인 강의를 세번 듣고서야 이해할 수 있어다. I could understand the online lesson only after I listened to it three times. Hope this helps. If you want to pracice speaking Korean and how to form sentences in Korean, you can visit the below link. https://youtu.be/wEpiNfED4PM

“-고서” Indicates that two or more actions occur in chronological order. Connecting ending “-고” + Connecting ending “-아서” are combined to “-고서”. “-야” is auxiliary postpositional particle for emphasizing. So “-고서야” indicates action emphasizes. “-고야” emphasizes the continuation or completion of an action or action because of “야” Connecting ending “-고” + auxiliary postpositional particle “야” are combined to “-고야”.