"There is a man with a knife behind the curtain."
Translation:Det står en mann med kniv bak gardinen.
"det finnes" can be literally translated as: "it is found", it is used as a fixed combination when talking about 'something being somewhere'. Like in French "il y a" (lit. it has there) and German "es gibt" (lit. it gives)
There is an apple. (=anywhere): det finnes et eple
There is an apple (=exactly there): der er et eple
"det finnes" can be even better translated as "there exists". Despite sharing the root, nobody thinks of finding anything when saying "det finnes", and i can't think of an example where it wouldn't be used in the context of existence.
As for your examples, you'd also include a "det" in "der er DET et eple", unless you're going for a very archaic, fairy tale vibe.
very very very generally, if something is taller than it is wide, it's standing somewhere, if it is wider than it is tall, it's probably lying there. Furniture can often be exempt (beds and tables stand on the floor, and if you tip a table over 90 degrees, you'd maybe say that it "ligger" (or "LÅ", because past tense?) there. If you said "det ligger en mann bak gardinen", the man would be laying down behind the curtain. certainly not quite as threatening.