"The jellyfish leaves the flæskesteg in the kitchen."
Translation:Vandmanden efterlader flæskestegen i køkkenet.
My take on that would be that "forlader" refers to someone leaving a place, i.e. the "thing" that leaves "forlader" that place whereas "efterlader" is like "leaves behind" in the sense that someone left, but is leaving something in the place that they are.
"Hun efterlader ham ved bussen" = "She leaves him by the bus"
"Hun forlader busstoppestedet" = "She leaves the bus stop"
Once again I wish I knew my own language better and actually knew what grammatical rules are in play here, but this is just something "one knows" if one grew up speaking Danish :-)
Note that it is often in "past tense" that these words are used, where they become "forlod" and "efterlod".