"메스는 수술대에서 떨어져요."
Translation:The scalpel falls off the operating table.
It should be 메스"가" 수술대에서 떨어져요 in order to say "the scalpel falls off / is falling off the operating table"
Because "메스"는" + sth" would describe a state/action/purpose or whatever that is usually true about the scalpel. So "메스는 수술대에서 떨어져요" sounds like "the scalpel [[always/usually]] falls off the operating table", and therefore sounds unnatural to the native speakers.
"메스는 + sth" form can be used in sentences like: "메스는 날카로워요 (the scalpel is sharp)" because it is true that they are usually sharp
..or it could be "(이) 메스는 수술대에서 떨어져요" which could mean "this specific scalpel tends to fall off the table"
other examples could be:
사과"는" 과일이에요: apples are [[always]] fruits
사과"가" 썩었어요: The apple is rotten (apples are not always rotten)
(이) 사과"는" 썩었어요: This apple is rotten (and will stay rotten)
Thank you for this! This is more insight on how to choose between them/particle dropping than I've gotten in a while. This is similar to the destinction between Spanish's 2 "to be" verbs: ser, estar. One is about permanently true things and the other is about temporary states.