In most sentences you could definitely use both and both would be correct! "Niemand" is always describing people though, whereas "Keiner (/Keine /Keins)" could also describe animals and things. For example: Keiner der Hunde hat mich gebissen. (None of the dogs bit me) In this sentence is "keiner" referring to the dogs. It's keine*R* because "the dog" (singular) in German is "DER Hund". This sentence would not be correct if you said "Niemand der Hunde hat mich gebissen" as dogs are not humans. In the sentence: "Niemand war dort" (Nobody was there) or "Keiner war da" (also translated to: Nobody was there) both words would be perfectly fine though, as "Niemand" and "Keiner" are referring to people. You could also say "Keine Menschenseele war da" (Not a single soul was there) and that would also be correct, but a bit more dramatic than the first two. In this sentence "Keine" would be used as "die Seele" (the soul) (singular) is feminine in German ("die"). On that note: even though you could use them both, "keiner" for humans is mostly used in situation where you expected certain people to be there / do something. It's likely to refer to a certain group of people that didn't appear or didn't do something. "Niemand" is used more often in situations where you really want to say "nobody" as in -> no human at all. This is only a general rule though, you can really use "keiner" and "niemand" synonymously most of the time! Hope that helped!